It is getting exceedingly hard these days to tell who is and who is not a Gray Eminence. Like bacon and hot dogs, men over 40 tend to go in for strong presevatives in a gallant efforts to fend off decay.

"I weigh less today than I did in my junior year of college," they gasped as they stagger in after a brisk jog around Rock Creek Park. From this one may conclude only that they were thoroughly repulsive in college, which, in fact, was occasionally the case. Gray Eminences tend to be reformed beer-guzzles, turned devouts consumers of bean sprouts and Grecian Formula.

To simplify matters, here are categories: There are young men who are, by and large, a tiresome lot. There are middle-aged men, by and large a tired lot. And then there are Gray Eminences. The Eminences are chronologically indistinguishable from middle-aged men. But they are also expert counterfeiters, having learned not only how to simulate youth, but to make it better than the real thing.

The way to tell Gray Eminences from younger men is by their backs. All of them are bad defy preservation, and cause them no end of trouble which, however, they do their manly best to hide by the simple expedient of denying there is anything wrong.

"My back is just fine, thank you," snarled a 46-year-old as he climbed into the sailboat he was about to race. He came in last, which put him in a bad humor. He also spent the rest of a Mexican vacation in a corset, which hardly improved matters.

The first thing all younger women have to know about older men is that they do not like being reminded that they are older. The second is that they will invariably grow huffy and mean-spirited when nature proves they are older. And the third is that they often wear corsets to correct the had backs they maintain they don't have.

It takes a little getting used to . . .

Young men fear death and failure, but these are not generally the chief ogres of middle-age. Older men - and here their bear a striking similarly to those shared by most women - have a horror of aging another minute more. Middle-aged men say succumb to inevitability; Gray Eminences defy it in a number of ways. When they have the flu, they deny having the flu.When they have a birthday, they ignore it. And of course, they date young women.

In the town that made Wayne Hays a household word, cross-chronological mingling is especially prevalent. Eminences like Henry Kissinger point to the aphrodisiacal properties of power. And in a way they're right. Washington is a town of young women who wish they were middle-aged men. But there is more to it than that.

Quite simply, the easiest and most painless way to acquire experience is to date it.

"Do you know when I first realized that I was an older man?" mourns a 42-year-old. "I was walking home one gorgeous night from work, and this nymphet across my path. I mean she must have been about 19, and she was lovely.

"She stopped when she saw me, and she said,'Gee, you look nice today."

But then she sort of paused and studied me. And after a second she said, 'No You're too old old for me.'"

Well, there are two points to this story. The first is, as the man says. "This all took place eight years ago. I mean I was YOUNG then . . . "

And his back was young then too, one supposes . . .

He looks up, outraged. "How did you know I have a bad back?" he demands, as the lady he has just married laughs silently. He turns on her. "You just wait until you're my age. You'll have a bad back too. Man wasn't meant to walk on two legs."

Which brings us to the second point. Eight years ago his new wife was about 19 years old.

No, actually, there is a third point. Around midnight that particular Eminence was told he looked tired.

"I will tell you when I'm tired," he muttered through clenched teeth. "And I also will tell you when I feel like going home."

The narcissism of the '60s, which declared unequivocally that over-30s were not be trusted, has in the end become rather wearying.

Young women were struck with mirror-images of themselves, the only sort of relationship the young men of that time would tolerate. It wasn't enough to be Left, one had to be New Left. It wasn't enough to be New Left, one had to court the like-minded. Ideological conversions occurred upon receipt of a first kiss.

"You know," he announced shortly after they'd met, "I honestly believe there's no difference intellectually between men and women."

They became engaged that night.

And, considering all this took place in 1968, theirs' was, in fact, a very tranquill relationship. They agreed on everything. They were, in a word, compatible. They were also, in another word, bored.

Within two years she was seeing a 45-year-old, a man her father was incapable of calling "Son," as had been his habit with her former boyfriends. A man who was, in fact, a Republican.

"That's a helluva way to bridge the generation gap," her father told her. But then he had once been a part of the Old Left.

Gray Eminences are, by definition, at least 12 years older than the ladies they are dating. When they talked about "the war," they do no mean Vietnam. They think Harry Truman was A Great president. They think Woodrow Wilson was A Tragic Figure. They mourn the Treaty of Versailies. They admire Bismarck.

When there is less than a 12-year hiatus between couple, the man possibly may be considered mature - but nothing more. Less than 12 years and youmuch of a feminist as you." And there isn't a Gray Eminence alive who would allow himself such a sentiment.

All Gray Eminences art, at heart and in other places besides, unabashed piglets viscerally, and egalitarians intellectually. Internally, this causes a lot of broken crockery. They have all had, at some time or other, wives whom they continue to support financially because women are hopelessly helpless creatures, and they will do everything in their power to keep them that way. But - and I say this with full knowledge of its consequences - that is part of their charm.

"Older men," says a woman now living with one, "are dominating, and I guess maybe there's a part of us that wants that.

"The younger men, you know, who have been through whole liberation process with us - they've been weakend by all that. While we've been strenghthened by it. And older men obviously like our strength. Except that they've disturbed by it, too. So we seek men as strong as we are. Or maybe stronger, I don't know!"

All Gray Eminences have the same problem: Although they are enchanted by younger women, they never quite believe that she is talking about. Knows what she likes or dislikes. Knows, in fact, what she knows - or, at least, is trying to discover for herself.

"Marriage???" sputtered a horrified veteran with two failed ones to his debit. "Why on earth are you thinking about marriage? I think the least you could do is learn from my experience."

"The older man I'm going with," says one young lady, "knows himself and how to make decisions. Which can be good or bad, depending on whether you want to make a decision for yourself. For instance, he doesn't want any more children.

"Now I probably don't either. But I don't want to commit myself on it. Or quit bagging him about it."

All young women have this problem. Gray Eminences firmly believe. Their problem is they haven't lived long enough. By "long enough," they mean they haven't survived as long as the Gray Eminences. On the other hand, had they done so, the Gray Eminences wouldn't be dating them.

"One of the reasons I broke up with you," a man in his late 40s confided to his young friend, "is because you're 29. Frankly, I've been 29 and, my dear, I don't really feel like repeating that tedious age."

The young woman understood that. What she didn't understand is why he had substituted her with a woman who was 26.

"You know," Sophia Loren informed a writer for McCall's, "between Carlo and me there are more than 20 years of difference and years of difference do count in a relationship between a man and a woman.

"I think Carlo considers me also his daughter in a way - and our quarrels reflect this . . ."

Gray Eminences quarrel in only one way: unfairly. When all else fails - logic, reason, common sense and, most especially, cool tempers - they will use their age as the trump card.

As in, "Yes, may dear, when I was your age I liked Hermann Hesse too."

As in, "Well, I imagine that when you get toLBJ/JKF/FDR/ James K. Polk/ did for this country."

These have the advantage of being absolutely irrefutable arguments. These also have the advantage of being overwhelmingly frustrating arguments. And these further have the distinct advantage of being rather specious arguments. As every Gray Eminence knows, one will never actually get to be his age. Or to be more precise, one will never catch up with him chronologically.

But then that, too, is part of their appeal.

And if advancing mortality makes them vulnerable, which it does, it also in an odd way makes them invincible, largely becasue they feel it is too late for them to change either their minds of their ways. This can have its drawbacks, but it is also aa distinct time-saver, as those of us who have wasted years fretting over the identity crises of young men cannot fail to appreciate. The one thing older men do not have a lot of is time.

For better or worse. I don't have many friends who share much in common with Sophia Loren. But I do have a close one who married an older man, which is why I asked for her opinion on the species.

"Well, they're sweet, you know," she explained. Then, realizing perhaps that this wasn't as analytical as it might be, she added. "I mean they know which battles to fight."