("Starting in the Middle" by Judith Wax: Copyright (c) 1979, by Judith Wax. Published by Holt, Rinehart & Winston. Distributed by the Los Angeles Times Syndicate.)

THE TROUBLE began in the early '70s, when suddenly a number of women I knew were being abandoned by longtime husbands. It wasn't just that it seemed sudden to me ; the stricken often reported that the epidemic hit before they'd seen the first symptoms. As with Legionnaires' Disease, nobody had the causes pinned down. But there were certain similarities among the afflicted: Most of the marriages had hit the two-decade mark; the children of these unions were old enough to be past traumatization and - by national custom - more likely to give shock than to receive it.

What many of the fleeing husbands seemed to want was new stereo equipment, fake leather furniture, and (this part made me nervous) very young women with long, straight, glinty hair, center-parted. The newly emancipated were given to trendy mannerisms of speech that unnerved their kids. A lot of them talked "grass," while some even smoked it. (Styled hair, like safari suits, was popular but optional.)

Separation and divorce figures have doubled in a decade for Americans 45 and older, and some gloomy experts predict it will triple in the next 10 years. If they're right, it means that one out of two marriages in the over-45 age group is on the endangered list. And though middle-aged husbandeds are still applying for exist visas in record numbers, during the past few years I was struck by how many women I knew all over the country who were clearing their throats and asking out. Not your basic young runaway wife who, like pregnant, unmarried movie stars, had a sort of pop vogue a few years ago. These women were in their 40s and 50s, and what seemed to be the single most arresting feature among them was their lack of rancor. Or passion! It would have been easier to make out what was happening if somebody had turned up the sound a little.

Certainly there are any number of women who are just now summoning the stamina to do what has long been contemplated, and for any number of substantial reasons. Harder to evalute are endings with curtain speeches that go something like: "You've been nice, it hasn't been all that bad. And what I want now is another life." The thing that's really hard to grasp is not the wrong or right of it, but simply . . . where do you get the heft to swing the ax if you're not really furious?

To say nothing of where do you get the nerve? Everybody knowas that as unattached men grow older, the pool of female "possibilities" increases. But despite well-documented evidence of women's sexual longevity, Ruth Gordon movies, and heady promises from certain fashion magazines, young men are not yet knocking each other unconscious in their rush to the first newly available middle-aged woman. Ripeness may be all, but it is not yet all that popular.

There's often a stunned quality about these deserted men, though none was more bewildered than my friend Bernice's husband when Bernic abandoned him for another nice 45-year-old lady like herself. "Did you know about her? Did you suspect?" he asked her old pals. But like him, none of us ever suspected that there was anything in Bernice's closest except her electric broom.

I like to think that I'm a kind woman (I like to think I'm voluptuous, too), but I couldn't work up much in the way of compassion for another deserted man I know, who once smugly informed me, "Any woman who expects to hang on to her husband damn well better have a face-lift by the time she's 40." The pronouncement sent me twitching to the bathroom mirror, and just as I'd suspected . . . my crow's feet had been turned to pterodactyl tracks.

Not that every rejected spouse is a chauvinist louse like that guy was. One, a neighbor of ours, had been a breakfast-tray bringer, supermarket schlepper since he married 30 years ago (all this and faithful, too). Nevertheless, his wife wanted out. "Space!" she wailed. "I need SPACE!" She wanted it decorator-furnished, too, and sent her dirty undies home after she'd moved out, for the laundress to deal with.

The night before she left, the couple stayed up till dawn while she explained the reasons divorce was a necessary step in her Human Potentialization. He cried most of the way through it - I'm not sure his pajamas are dry Yet . However, just as the sun came up, he though maybe he'd begun to understand at last what had gone wrong. "Tell the truth," he sobbed, "have I been acting too macho?"

Before my old friend Francesca's marriage expired, it was one of those old specimens people like to trot out as a last bit of perfection in a world gone putrid. The problem, Francesca said, was that after several decades of alleged perfection, she found herself no longer able to get up and put her clothes on in the morning.

Husband Super Bowl

When I was a child, my grandmother and I were this country's leading authorities on "The Romance of Helen Trent," and every day we'd huddle at the radio while the announcer explained that the program was decidated to proving "what so many women long to prove in their own lives: that because a woman in 35 and more, romance in life need not be over, that the romance of youth can extend into middle life and even beyond" [emphasis, the announcer's]. So, though I probably have my grandmother and Helen Trent to thank for my silly romantic streak, it's also due to them that my research - however unorthodox - into what the mature woman is up against has been going on since before I collected movie stars' autographed pictures and even before I owned a Charlie McCarthy spoon. I know that if a woman "in middle life and even beyond" is looking for romance today, she's probably learned to shut up about it. She'd probably, as I do, agree with Doris Lessing that it's a waste of a life if you hang about just waiting for a man to confer meaning on it. It's just that there's so much else for my moonlight-and-rose-nurtured generation to learn and unlearn.

It has to mean something that every one of the women I'm talking about married young. My generation was dispatched to college as if it were the Husband Super Bowl, and when we didn't know what to do about our feelings - at least our sexual ones - we legalized them. It's well known that Margaret Mead proposed that young couples considering marriage should live together under a contractual two-year legal arrangement after which they'd mutually decide whether to marry or simply dissolve the bond. The idea came a few decades late for most of us, but maybe she could have thought up some kind of National Plan for Old Marrieds, too. Maybe there ought to be legal reststops at, say, the 15th or 20th anniversary, when husbands and wives could elect to separate for a few months and then come back together - no questions asked - and decide whether to cancel or renew.

I admit the details could get tricky, so I leave it to others to work out things like where one goes, what happens to jobs and to dependents so tactless as to still be hanging around the old nest. Maybe Milton Friedman could work out a plan for finances. Okay - jokes, jokes. But it does seem that the late-in-marriage breakup has a lot to do with, perhaps, only a temporary need to taste that world out there. I bet most people would settle back together once they'd had a few licks at it. And it probably all works out a lot cheaper than lawyers and alimony.

The big word today is choices, of course. The catch is that nobody has really refined the distinction between the choices that lead to (watch out: jargon ahead) self-realization or self-destruction. At our age, one ought to get a chance to find out which is which, so we can make an informed decision about whether to crawl back from the edge of the abyss while there's still time or jump. . .I confess that's a little like asking, "If I make my own bed, do I still get to lie down on percale sheets?" but so what? Even that dummy Goldilocks knew enough to test the mattresses.

Smorgasbord of Sex

I can only offer the slenderest, most empirically fragile data from my wanderings among my peers and my own mid-life confusion. I know that when a 48-year-old woman who lives alone and doesn't date says, "I never feel as lonely as when I was married," it's probably lucky she worked up the nerve to do her Nora number. I know from the one who's collecting Seconals now instead of milkglass that boredom has to be better. And I don't think anyone can predict how many of us who walked out will prove to be happier and sounder for it and how many will have been victims of one season's fashion that turned into next year's rummage.

I, for one, will continue to poke around for portents, like an ancient soothsayer looking for signs in the entrails of sheep. What clues are there, for instance, in this postcard from an old friend who married as I did before we'd even heard of stretch marks? "since Stanley let me have the divorce," she writes, "I'm finding my center in Tai Chi, self-exploration, depth-plumbing, seaweed, a soupcon of chemicals, a smorgasbord of sex. What's new with you?"

I knew her when she wore dress shields.

Hearts and Jello-O

My queenly bridal gown has been preserved in pink tissue for 27 years now. The matching satin shoes are nearly virginal, and so was I the day I teetered down the aisle in them. I took the old gown out and stared at it a few days before our lastest anniversary. It unnerved the silver-haired groom, who though I might be plotting one of those renew-the-vows ceremonies you read about . . .where midlife couples get into their ancient wedding attire and summon friends, relatives, and progeny to witness romantic history reenacted. He needn't have feared. I had disinterred my gown because I was thinking about old marriages, wondering why some stay happy, what my own was really like. It was thorny stuff, and I suppose that staring at the gown was my idea of shcolarly investigation. Besides, I'd finished my research intto whether, if you listen closely, you can hear your own arteries hardening, and you can't)

I do suffer from virulent attacks of sentimentality, a kind of St. Valentine's dance that compels me to throw an annual Feb. 14 fit of a party. During one February seizure, I rigged out our dog as Cupid. He ate not only his left wing, but all the pink candles on the coffee table and delivered vomited valentines to us for weeks. Another February, I made the ruffle-necked dress of red-and-white heart-patterned fabric with an under-the-bosom sash that cries out for Tricia Nixon. When I also ran up napkins and cafe curtains in the same material, my husband offered to hang me at the window so that arriving guests could play "Guess Which Is the Hostess and Which Are the Draperies."

I met my gray-haired valentine 29 years ago. I was an 18-year-old college freshman; he was 22, an ex-GL senior whose eyebrows - I marveled - looked just like Tyrone Power's. We "went steady" from the first date, were duly "pinned" and engaged, and I married him two years later only partly because he explained (in the dark garage behind a feshman dorm), "Sex is an integral part of life." Nobody had ever said "integral" to me before.

You don't have to consult statistics to know what happened to many of those unions that grew out of first-blush college or even high-school romances. Still, I can only really address myself to the union that began with a campus decision I once made half of. I try to think up reasonable explanations when people ask why this particular brontosaurus is still munching the leaves in the garden of wedlock. And people to ask.

Sure, we're had some terrific days; whole good years, even. But if Shel and I ever sat down and tried to figure out which of us did what to make this marriage heave, inch, careen, gasp, exalt, despair, lumber, leap, shine, bore, delight, and rattle through 27 years (plus two premarital ones of a little fooling around that ny self-respecting modern kid would choke - laughing - at), we'd probably end in a terrible, terminal battle.

Everyone knows, of course, that merely sticking together isn't proof of the worth of the bond. Consider all the paste-up jobs that Should have come unglued. Maybe worthwhile durability can only be explained by some of the marital milestones. Maybe, instead, the working marriage can only be glimpsed through the sum of its non sequiturs. There isn't a nice mathermatical formula for evaluating the cumulative effect of all those isolated moments of marriage-in-progress, nothing that totals it all up neatly.

Perhaps, with the tempering of age, I'll learn to control the romantic reflex and come up with nice solid scientific data on The Living Marriage; with luck, there'll still be life in mine. Meanwhile, I promise never to appear at one of my valentine parties in the old bridal gown that is holding up better than the old bride. Not that I'll ever kick the valentine habit entirely. But next time - with God's help - maybe I'll be able to restrain myself from making lots of little heart-shaped Jell-O molds.

The Affair

There are other options besides ripening together in a mellow old marriage or running out on what's gone rotten. One of them - no doubt the word has gotten around - is the affair. But what with the years spent in child-community-and-kitchen craft (and paying attention to Dr. "Can-This-Marriage-Be-Saved?" Popinoe), a lot of us didn't discover the possibilities of the double life until we'd been hit by the possibilities of the double chin.

Which doesn't mean the late affair has to be the stuff of farce, a last-chance cavort of aging flesh. Many women say that for them, it has meant lovely erotic discoveries, or rediscoveries. Some say the lovely eroticism was discovered because they forgot their assigned places in our youth culture, forgot all that had been carefully learned, and fell astoundingly, extramaritally in love.

For some lucky people, beauty was conferred for all time in marriage that remain impervious to other loves and other beds. (You don't qualify for either eternal imperviousness or virtue, though, if there was never been a serious challenge by someone really tempting.) However, if love doesn't ever reveal itself to us, we aren't above pretending there's no such commodity anyway. Which explains why a monogamous woman can hurl herself into one first fling with a man she's not all that crazy about . . . just to get the flavor of the general experience before the taste buds go. Besides, the same women's magazines that used to dish up "50 Ways to Stretch Hamburger" as if it were the wildest reaches of self-realization are now running articles with titles like "Your Affair May Help Your Marriage" and "It Is Possible to Love Two Men . . . Your husband and Your Lover." They even offer surveys and statistics that show you don't die of adultery (although the marriage mortality rate seems about 50-50).

Such articles might wind up ending infidelity in America. Who's going to want to savor forbidden fruit when homemaking magazines are outdoing each other with recipes for Illicit Compote?

Norma, who has been faithful to her husband for the 21 years they've been married, confesses that she has been readying what she calls "my adultery underwear wardrobe" for nearly as long. "I'd start fantasizing about some man - for years it was my gynecologist, and for a while a man on my Save Our Trees committee - but nothing ever happened. I used to comfort myself that I probably seemed so unapproachable, so dauntingly wholesome that no one dared kiss Sleeping Beauty. But never mind, I was getting this lingerie wardrobe together just in case there was an aging Prince Charming somewhere around who had the nerve to break my spell."

Several wives I know broke their own long-standing records for monogamy by looking up "the man I almost married."

"I was the last one left in my book discussion group who hadn't had an affair," one told me," And so I went stalking the only person in my whole life - other than my husband - who had ever asked me to sleep with him. I did it in cold blood, but I made sure to undress under the covers because the last time he asked me. I was 22 years younger and nearly that many pounds lighter. And you know what he said? He said, 'I waited 22 years for this, and now I find out you're just like my wife, a closet undresser'."

A girlhood suitor was my friend Pauline's choice, too. "It seemed the safest way to try it at least once before I die," she said. Pauline was spooning strained apricots into her first grandchild when she told me about her adventure. "Before I married, my idea of being a 'good girl' was to take one more step with each of a succession of boyfriends. So the last boyfriend I had before I married was the one with whom I'd do everything two bodies can manage except actual intercourse. That's why it seemed only the correction of a technicality to call him up 30 years later. And three days after that, we were together in a hotel room."

Pauline wiped some sneezed Junior Apricots off the glasses that hung from a chain on her still-graceful bosom. "I suppose that knowing the man had once wanted to marry me made it okay, in the family, so to speak. After all, if 30 years ago my idea of morality was that I could sleep with the groom-to-be once our wedding inviations were mailed, surely I can be flexible enough - I mean update my moral code enough to include the man I could have married if I'd wanted to. The amazing part is that he actually came all the way to Chicago from Detroit when I called him up, after all this time. I can thank his wife for that. The reason I knew where to reach him is that she's the kind of woman who's secure enough to send Christmas cards to her husband's old girlfriends."

Whatever the conflicts or rewards of late discovery, major handicap is often lack of training. One has to get it on-the-job, so to speak, and a lot of us turn out to be more naive and less experienced than our youngest daughters. So you pick up hints where you can. If you've read Colette's "The Vagabond," there's instruction in the heroin's remarks about certain women who are "at the age of fatal imprudences." She says, "I have shuddered at the lack of awareness of a friend in her 40s who, unclothed and all breathless with love, clapped on her head the cap of her lover, a lieutenant of Hussars."

But a Philadelphia woman I know hadn't read Colette, so in the exuberance of her first affair she put on the adored one's Stetson and marched her 44-year-old nakedness around the hotel room during one of their regular Thursday afternoon trysts. She was doing an electrifying baton-twirler imitation, she says . . . around the room, over the rumpled bed; she even leaped onto a chair. (She also had drunk more wine than all those years of bologna sandwich lunches in her kitchen had conditioned her to handle).

When the nude majorette glanced over at her playmate, his grin had jelled into something less than admiring. And though wife is a woman celebrated for the fact that nobody has ever seen her smile, apparently he needed someone who could be counted on not to do hat tricks, and he returned to his legal lady, presumably forever, including Thursdays. Had he stuck around, he might have learned a lttle about the joys of laughing in bed; no doubt the Philadelphia woman is well rid of her stuffy lover. On the other hand, she'd grown painfully fond of him. And the husband who now gets all of her attention wouldn't notice, she says, if she wore a hat to bed or marched across it leading a drum and bugle corps.

Another harzard of the late start is the one's skills for gauging a potential partner may have been put to the test once - and not for several decades. So if you're just taking the game up, you can find that you're dangerously out of practice, which is why when some women fall in love, they also fall for lines that ought to be pelted with rotten vegetables.

A first-affair dropout reports that she knew the line she fell for ("My wife and I haven't had sex for years, and you're the first woman I've ever loved") was as old as "Twenty-three skiddoo, Chicken Inspector." That's exactly what was so brilliant about it, she says. "I didn't think anyone would have the nerve to try such an antique unless it were true.

"Well, lying to me about his wife was one thing," she says. "Telling me he'd had a vasectomy was quite another. And quess what middle-aged woman found herself pregnant? My husband has had a vasectomy, so I couldn't tell him. I couldn't go to our doctor, either because he'd know it wasn't homegrown. So one nice morning I hauled myself to an abortion clinic, and there I was - 42 years old and never an unwanted pregnancy in my whole life - waiting with a bunch of scared-looking teen-agers. I looked around at those poor silly kids, and I knew I was the most childish one there. I also cried the hardest; this one little blond girl, no more than 15, came over and put her arms around me and cried, too. But her boyfriend was in the waiting room, and mine - when he heard the news - mumbled something about he thought we were too old to be fertile. . . and took his wife to the Pconos."

But just as certain women's magazines - and some of my friends - say, there are marriages that were expiring until somebody breathed new life in the old wife. I've talked to women who swear that for them, infidelity turned out to be as restorative as the Heimlich maneuver. The happiest nouveau adulteress I know is my girlhood friend Greta. She's 46, married 20 years, and landed her first job - and first lover - two years ago. "I can't wait to get up in the morning," Greta told me, "and sometimes I can't wait to get back to bed the same aftenoon, which is one of the advantages of partime employment! I know it's crazy, but my marriage has never been happier, and I honestly don't know which of my two men I love most - a fact neither of them could handle if he knew it." CAPTION: Illustration, "It seems only yesterday I was OK, you were OK." By William Hamilton; Copyright (c) 1974, The New Yorker Magazine, Inc.; Picture, about the author