She, at 43, had just separated from her husband, and was living abroad.

He, at 21, had enrolled for a semester at a European university, and was alone.

They met, and -- as is often the case -- first became "excellent friends" and then lovers. Though she was old enough to be his mother, and indeed had a son older than he, their relationship flourished for more than a decade.

That it lasted so long, she says, was not because of the age difference "but in spite of it."

She was hesitant initially -- knowing how society titters at the idea of an affair between an older woman and a younger man, meanwhile accepting the April-November marriages of a Supreme Court justice and a U.S. senator and their young brides.

"Don't be ridiculous," she told him at semester's end when, as she puts it, "he declared himself." She packed him off to the United States, but his letters kept coming.

The letters convinced her. "I decided he must be serious." He flew back, and they spent the first of their many summers together.

Increasingly, we are reading about marriages, or affairs, between women who are 5, 10 or 20 years older than their male partners. To cite a few names that have been linked at one time or another: Singer Dinah Shore, now 63, and actor Burt Reynolds, 44. Actress Louise Fletcher, 44, and James Mason's son, Morgan, in his mid-20s. Actress Jeanne Moreau, 52, and director William Friedkin, 41.

Because many of these reported unions emerge from Hollywood, the question arises whether this is just one more crazy movieland fad.

"It's a life-style thing among the beautiful people," says one author who is writing a book on the subject. "They're not ordinary middle-class people doing it."

Or, as others believe, are we possibly becoming more open-minded about such relationships?

If the First Family helps set the tone for the nation, that may be the case: Two Reagan offspring are involved in this life style. The president-elect's son Ron, 22, recently married 29-year-old Doria Palmieri. And Ron's half sister, Maureen Reagan, 39, is engaged to Dennis Revell, a 28-year-old lawyer.

However accepting society may be, the older woman-younger man relationship remains something of a rarity. Yet those who have been a part of one -- for even a short time -- speak glowingly of their experience.

And they speak of the problems that can eventually break up the romance.

Says the 43-year-old woman whose relationship lasted for more than 10 years:

"For the woman," an affair with a 21-year-old "is assumed to be sexual -- the older woman seeks to restore her youth in the arms of an Adonis. But for me, that was the least of it. Naturally, there is an erotic impulse but what we got didn't have to do with our age."

Instead, it brought "companionship. We enjoyed each other's company. He has a first-class mind.I am witty, and he tends to enjoy that." Even in simple housekeeping chores, "We worked well together. We were profoundly close companions."

Though her income was greater than his, "we always shared equally." When he had to borrow, "he would pay it back."

Her lover, she acknowledges, was not typical of men his age. "He felt alienated from his own generation -- a little square, reserved. He felt all the girls were nitwits. He valued my mind.

"Of course," she says, "it was flattering" that he also considered her beautiful. But she knows their relationship was built on more substance. "He was not a callow kid.

"It's flattering to the young male, too, to have an older woman think he's interesting."

Once, she says, "he suggested marriage. I think he was very gallantly trying to do the right thing. I was able to talk him out of it immediately. It is too dangerously an unstable way to live, and we're too sensible."

That cautionary note pervades her recollections.

She feels strongly that there are good "genetic" reasons why unions such as hers are considered something of a "taboo." Older men can readily father children by younger wives, but in a younger man-older woman relationship, the chances of conception diminish rapidly.

Other factors intrude on the romance.

"For conventional people to live as an oddity," and she places the two of them in that category -- "to be ever-defined in that role -- puts a strain on the relationship."

At the same time, a couple "when they love each other, even they will assume as a woman gets older she will lose her young man to a younger woman. Any sensible couple in this bind will assume that eventually the woman is going to be less attractive."

While relatives initially were approving of their relationship, after 10 years "there was a tremendous rift in his family. It was quite tragic."

That, she says, was "one of the reasons I said I can't go through with it."

If that's how an older woman views this kind of relationship, how does a younger man see it?

A then-20-year-old Washington law student home for the summer met his former high school teacher, who was 30, at a shopping mall and invited her to lunch. It was, he recalls two years later, the beginning of a 10-month affair that had a profound effect on him.

"It makes someone that age grow up a whole lot faster. You're with somebody who is pretty stable. They're more at peace with themselves. It rubs off on you.

"I think she depended on me for emotional stability, too. She was in the aftermath of a crisis, a divorce."

They tried to ignore the age difference as best they could. "We didn't talk about it, though at times it did come up. When we discussed the '60s, I'd be talking about Mickey Mouse and she'd bring up the peace rallies. But we didn't use it against each other."

Their families and friends were accepting. "My mom had a surprise birthday and invited her. Some of my friends were her ex-students."

One friend, he says, joked to her: "I knew he was your favorite in class. But this is ridiculous."

"We dealt with each other as two people -- not as teacher and student. She didn't look at me as a 21-year-old kid, but as someone who had known her in a different stage. I had known her for five years."

A psychologist friend, he says, told them that since they both were at their sexual primes as male and female, their relationship "was made in heaven physiologically."

The question of marriage, he says, did not come up. "She'd just been through one. And she was considerate of the fact I was still in school." In time, his being away at school led, as much as anything, to the end of the affair.

A 40-year-old Washington writer, who at 26 had an affair with a woman eight years his senior, found the relationship "comfortable."

"By the time a woman reaches 30, she has the bruises. She knows what to expect of life. She's not as -- 'flighty' is not the word -- apt to expect more out of life than a man can ever give.

"She comes to terms with life. The relationship has more of a warmth. It doesn't have that intensity that's hard to deal with -- but it can have incredible passion.

"As a young man, you learn a lot -- physically, sexually as well as emotionally. It's fun to be taught instead of always expected to be the teacher.

"It could have lasted," he says, "but I wasn't ready to take the steps."