It was the issue that didn't come up about the man who didn't show up. The shadow subject at the Iowa debates and in the whole election has been Ronald Reagan's age.

In a few short weeks, Ronald Reagan will be 69. He'll be old in an era when we insult age the most by covering it in euphemisms: senior citizens, sunshine years, the golden-agers.

So far, his Republican opponents have virtually declined to mention what everybody knows. They deal with age as if it were a birthmark instead of a birthday.

I suppose they are being politic as well as polite: no candidate wants to be accused of agism. One man may be ancient at 69; the other, Winston Churchill in the midst of World War II.

But I suspect that the candidates are also uncomfortable with this subject. They are, after all, mostly in the 50s, only one peer group younger than Reagan and uncertain about their own futures. Perhaps they, too, would want to celebrate 70 with an inaugural. o

This uncertainty about aging isn't reserved for politicians. Our whole society is aging, and few of us know what we want to be when we grow old.

Our life expectancy today is 73.2 years -- 69 for men and 77 for women. If it continues to rise at this rate for 20 years, by 2000 the average life of a woman will be 84 and that of a man will be 74.

Looking at our own future, I suppose most of us hope that there must be a way to age gracefully, to move into another stage of life with more style than we could muster up in adolescence. But at the moment the greatest compliment that the young pay the old is to say, "You don't act your age."

There seem to be two dominating role models: those who behave as if 70 were really 55 extended a bit, and those who have retired into emptiness. But we have too few people acting as contended, guideposts, sending back positive messages of what is ahead.

A Harvard graduate I know came back from his 50th reunion last June with some new insights and observations. There was something different about this reunion, he said, different from the 25th -- or the 40th, for that matter. The old divisions of background, occupation, success and class seemed to have softened, fuzzed through the spectrum of age. The men were kinder and more open with each other.

In part, he attributed this to their feelings as survivors. But also, he noted, one of the benefits of old age may be in letting go of ambition, competition, status-seeking.

I would rather look forward to this mellow notion than to spend my 70th year on earth striving to spend the next four years in the Oval Office working 16 hours a day.

But I am also aware that many older people feel not mellow but useless in age. There is something absolutely terrifying about those elderly who spend the last time they have left killing time. It isn't only George Burns and his cronies who rebel at the notion of spending the "golden years" bored.

My own fantasies about aging are probably as unrealistic as my young fantasies about mid-life. But they lie somewhere between restless ambition and emptiness. The first, it strikes me, is unseemly; the second is surely premature death. Yet they are the two options for the aged we hear the most about: holding on to mid-life and letting go of all life.

I suppose I fantasize a kind of contentment in which it is possible at last to be engaged and yet somewhat detached, involved but with a perspective, and accepting.

Today the elderly are usually the subjects and not the authors of tales. Our senior citizens are written about as problems, and our "Passages" are supposed to stop at mid-life.

The difference between me and my daughter is that I have been both her age and mine. The difference between me and the elderly is that they have been my age and theirs. There are now 24 million Americans over age 65. When I am 65, it is likely that there will be more than 32 million.

We need to hear more about aging from those who have aged well. We need some people from the generation that taught us how to grow up to begin teaching us how to grow old.