Last September, the 25th anniversary of the founding of the Peace Corps ended with a Harry Belafonte concert at the Kennedy Center. Belafonte sang his signature song, "Matilda," and, as he has done hundreds of times, organized the audience to sing the chorus. He had the balconies sing one chorus, the orchestra level another, men one, women another and, finally, "women over 40." There was a slight hesitation, a cautious beginning and then, full- throated, several hundred women sang out: "Matilda, Matilda -- Matilda she take me money and run Venezuela."

Belafonte stopped singing. He was stunned. He said he had sung this song hundreds of times, all over the United States and around the world -- "even in Africa" -- and not once had women over 40 responded to his request. That, of course, was the whole idea. It was a joke, and it always brought a laugh. This time, though, the joke was on Belafonte.

Of course, former Peace Corps volunteers are not typical people, and, I suppose, that's especially true of the women. They are people who once volunteered to live abroad for two years, often in a Third World country and often in the countryside. They willingly accepted hardship, often disease, because they combined, in one proportion or another, a yearning for adventure and a desire to help less fortunate people. It turned out that some of them were, years later, still bold. They had the guts to acknowledge their age.

I have never quite understood the reluctance of some women to admit their age or, worse, their impulse to lie about it. When Gary Hart did that -- misstated the year of his birth -- it troubled many people and raised all sorts of questions about candor and character. Yet women do it routinely and, usually, with impunity. It's as if on this score there is a sexual standard of honesty -- one for men and another for women. What would have happened if a female presidential candidate had been caught lying about her age? Could she have excused it with a giggle?

Amazingly, this deceit has survived the women's movement. While the vanguard of the baby boom, now around 40, proclaims its age and celebrates its middle-age heroines (Jane Fonda, Gloria Steinem), many other women still cling to age deception, an anachronism like the decorous faint or inappropriate tears. One of those women, apparently, is Nancy Reagan, who has been accused of misplacing a couple of years. She is either 64, according to the White House, or 66, according to her college records. In either case, the two years are without significance. The lower age hardly makes her a young woman; neither does it change reality. In that sense, age deception is not like makeup. Mother Nature keeps an accurate count.

Mrs. Reagan's (suspected) chronological sleight of hand is certainly not uncharacteristic of her. She is no feminist. Not so Molly Yard, the newly elected president of the National Organization for Women. She is indisputably a feminist. Yet a New York Times profile of her written right after her election contained this surprising phrase: "Ms. Yard, who is in her 70s but steadfastly refuses to divulge her age . . ." Why? We are not told. Possibly it's out of vanity. Possibly Yard is reacting to yet another prejudice: ageism. Either way, she has allowed her response to be dictated by others. She is clearly of a certain age, and there is no reason not to say what that is.

What's distressing about the insistence of some women to hedge about their age is that they then become fellow travelers in the movement that holds youth and attractiveness to be synonymous. We all know that women and men are perceived to age differently, that for some reason wrinkles are said to give a man character but to turn a woman into a hag.

Cary Grant and Clark Gable remained leading men into their late fifties while women of the same age became character actresses or, like the 49-year-old Fonda, heroically keep the body of a 20-year-old by maintaining a fighter's training schedule. Joan Collins, yet another woman who admits to a decade and not an age, has triumphed in small-screen television, but it's inconceivable that, like Paul Newman, she will be playing romantic leads at 62. Greta Garbo apparently felt compelled to retire at about half that age and Brigitte Bardot hung up her pout at an age when some ballplayers are still taking the field. Only a few actresses can still find work once their youthful beauty is gone. Katharine Hepburn is one. Through sheer force of personality, she is as beautiful in "Rooster Cogburn" (1975) as she was 40 years earlier in "Alice Adams."

But if anyone has an obligation to say -- no, announce -- that the worth of a woman consists of more than looks (and youthful looks at that) and that attractiveness consists of more than a face off the cover of Seventeen magazine, then it is women themselves. If only out of self-interest, they ought to announce that, like men, they also get more interesting with age -- that they get wiser and more sophisticated and, maybe, richer and more influential as well.

This, I think, is what the former Peace Corps volunteers were saying when they sang out at Belafonte's invitation. They were women, not girls -- women of accomplishment, of experience, of self-knowledge and certainly some other kinds as well. They were, in short, who they were, and part of that was their age. It was, truly, something to sing about.