We're getting older. As the baby boom contends with mortgages, diapers, baldness and midriff bulge, America is becoming middle-aged. The themes of middle age creep into our songs, literature and television shows. The baby boom -- the generation born between 1946 and 1964 -- accounts for nearly half of all adult Americans. By sheer size, it dominates (perhaps unfairly) national tastes and preoccupations. Its aging is a momentous event that affects everyone. But how? That's an awesome puzzle.

Is our middle-aged society bound to be more politically conservative? Does middle age portend a loss of imagination and economic vitality, ultimately leading (as Ben Wattenberg of the American Enterprise Institute worries) to a nation of "old people in old houses fondling old ideas"? Are we sliding into a collective midlife crisis of dashed hopes and career frustrations?

All these plausible guesses may be wrong. Only the dry trend lines are clear. There was much hoopla last year when the baby boom's first members turned 40. The more important transition occurs this year as those born in the boom's peak year (1957) turn 30. Nearly two-thirds of its 77 million members have now passed the psychic barrier that demolishes the pretense of adolescence. By 1995 the number of Americans between the ages of 35 and 55 will have risen by a third. The nation's median age, 30 in 1980, will then be 35.

Of course, the baby boom is no monolith. It reflects all of America's diversity. But age does inflict some similarities. Middle-age culture is "more stable, serious and careful," as Cheryl Russell, editor of American Demographics magazine, writes in a new book, ("100 Predictions for the Baby Boom.") It's already beginning to affect the nation's mood and concerns. Russell thinks middle-aged moderation lies behind the public reaction against drinking and driving, drugs and pornography. How else will the aging of the baby boom change society? Three speculations:

Higher school spending: More than anxiety over "competitiveness" explains the new preoccupation with education. As members of the baby boom have children, the constituency for better schools grows. In 1985 public school enrollments rose for the first time since 1969. They're expected to increase by about 15 percent by the early 1990s.

Less leisure: For the new middle-aged, juggling demands of work and children with personal pleasure will make life more harried. Convenience products will flourish. Consumers are just as time-sensitive as they are price-sensitive, argues demographer Peter Morrison of the Rand Corp.

The death of youth culture: An aging society doesn't want to be reminded it's getting older. "We're going to see more advertising showing middle age in a positive light," says marketing consultant Judith Langer. The de-emphasis of youth will also reflect the fact that there will be fewer young adults. By 1995 the number of Americans between the ages of 20 and 34 will drop by nearly 6 million.

The contemplation -- and celebration -- of middle age is already infiltrating popular culture. Is it mere coincidence that two family sitcoms, "The Cosby Show" and "Family Ties," dominate the TV ratings? In the 1970s "The Mary Tyler Moore Show" (focusing on singles and work) played to a then younger audience. Country singer John Conlee recently had a hit called "Domestic Life." Its hero is disheveled and disorganized. His car is falling apart. He dashes between work and Cub Scout meetings. He's overloaded with debt. He loves it. The chorus goes (in part) like this:

Mowin' my domestic yard

Lord I owe my soul to

MasterCard

But it seems to suit me to a tee

That domestic life's alright

with me*

It's Norman Rockwell put to music: so corny it's charming. There are more somber themes. Moe Bandy, another country singer, had a hit about a father facing the inevitability of his own death and praying that he will live long enough to see his children grown.

What's evolving is not a reversion to 1950s life styles or values. It's a bewildering blend of traditional values -- home, family, work -- and more recent expectations. Russell of American Demographics argues that members of the baby boom may be aging, but they haven't developed amnesia. Their attitudes and experiences often differ profoundly from those of their parents. Divorce is acceptable; two-earner couples are the norm. Conflicting values abound. For example, "The importance of work is tempered by ... the search for instant gratification," says Russell.

Political stereotypes don't fit comfortably either. The labels "liberal" and "conservative" no longer describe the ambivalent views of many Americans. The vast middle of U.S. politics is a muddle. Suspicion of government mixes with expectations that government should promote stability and progress. By one poll, three-quarters of Americans think government "wastes a lot of money"; by another poll, roughly 60 percent of Americans think government should spend more on education, health and the environment.

We are awash in political and personal contradictions. Will they breed discontent? Probably. But discontent has its uses. Indeed, the tensions may keep an aging society younger. The midlife crisis may be an antidote for boredom -- a spur to career changes or new ideas. When Wattenberg speculates about the flagging economic energies of an older society, he's admittedly peering into the 21st century when (if current birthrates continue) the aged population will be much larger. But no one knows what else may happen between now and then. It's too far for anyone to see.

The only certainty is that the ordinariness of middle age won't suit a generation that demands to be different. Middle age is bound to be relabeled, if not reinvented. Well, why not? The concept of middle age, as Yale historian John Demos has noted, is relatively new. For most of our history, people pondered only the problems of youth and old age. The period in between was considered the prime of life. Who knows? Maybe they had it right.

*Copyright 1984, 1987 by Music Corp. of America Inc., New York, N.Y., and Dick James Music.