"GROWN-UPS" A Generation in Search of Adulthood By Cheryl Merser Putnam. 237 pp. $17.95

What is a "grown-up," anyway? Is it someone who has reached 21? Does it have something to do with acquiring wisdom or shouldering responsibility? Is it a state conferred by marriage or childbearing? And why is it that some men and women seem to take a lifetime to leave childhood behind?

"Grown-Ups," Cheryl Merser's study of baby boomers' peculiar maturation patterns, focuses on why it is so difficult for many rapidly graying men and women to feel -- or even to act -- as if they are "grown up." The post-World War II generation is unusually rudderless: They have entered their middle years with few of the rites that previously forced maturity on the young. Today, for instance, baby boomers seldom take their cues from religious definitions of adulthood. Childbearing, usually a rite of passage into female grown-upness, can now be delayed until the fifth decade of life, or avoided entirely. And marriage has proved as mutable as most other traditions: A 50 percent divorce rate, the influx of women into the work force, and feminism have turned marriage into a constantly renegotiated contract.

The young today can't even take many cues from their parents' coming-of-age patterns. In her most interesting chapters, Merser explains that baby boomers' parents were a demographic anomaly. Soldiers returning from the war married early, and they and their young brides embraced adulthood with a precocious fervor. Twenty years later, with their children grown, this generation, which had spurned youth when it was young, rediscovered it with a passion. For these early bloomers, midlife crises often resulted in men, like Merser's father, rejecting their staid jobs and homes for a more daring, more sensual middle age. Meanwhile, their wives chucked their aprons in the trash, got an education and entered the masculine world of work.

Left no useful legacy of what it means to be adult, Merser's generation has clung to youth, while playing grown-up. More than any previous generation, Merser and the friends she quotes have made a career of job-hopping and apartment-hopping, enjoying live-in lovers and long- and short-lived friendships. For many, there will never be a spouse or children. For some, no responsibility will be taken for any living creature. Can this be a new and legitimate path to adulthood?

Unfortunately, Merser's answer to this central question is a resounding "sort of." "Neither childhood nor adulthood is absolute," she writes. "It's {likely} that we slip back and forth all our lives from the consciousness of an adult to that of a child, as we struggle to overcome our fears of the world and make peace with it."

Merser is an entertaining analyst of her generation, but her generalizations about her cohort of young adults are often unconvincing. She extrapolates frequently from her own and her friends' years as single, urban professionals, ignoring the vast majority of her generation who still marry at 22 and have their first babies a year later. She has no explanation for the 30-odd percent of married couples who follow their parents' example, with male breadwinners and wives at home full time. More disturbingly, Merser bends her argument into a pretzel to convince the reader that a life without real responsibility is as sure a path to adulthood as the more traditional ways.

For a rarefied minority, of course, Merser's analysis fits like a glove. And even the most ardent youth-groupie does eventually grow up. Merser, childless, ending a long-term relationship, experienced the critical break with her preadult self when she got a dog to look after and discovered the yuppie equivalent of Eriksonian generativity: the "human longing to be the 'hero' of my life, to justify my existence, to reach for immortality. Caring for Phil {her dog}, wanting a child, maybe writing this book, and in various other ways, I must be doing what we all must do to become adult, to prove that we're human, to prove that our own reality connects in a meaningful way with others' and won't simply die when we do: I must be trying to make an offering to the world in recompense for what the world has offered me."

And how is this new awareness put into action? More with a whimper than a bang: "I think more about life," Merser writes, "my own life and the ways in which it touches others' or doesn't. I watch over Phil, and I worry about friends who travel on holiday weekends. I have a young niece now, and I want her to be safe. I worry about my parents, not yet old, and I worry about people I don't even know -- abandoned children, the homeless, lonely victims of AIDS. This is not to imply that I've become saintly or unaccustomedly generous: My idle worrying amounts to very little ..."

The problem that Merser rarely faces in this book is the immaturity and selfishness of men and women who can worry, a bit, about suffering, without bestirring themselves into action. Throughout this interesting book, there's a strained quality to Merser's eagerness to assert the primacy of individual self-discovery over traditional bonds like marriage, and her effort to redefine family ("The point is that we're all 'starting' families, all our lives.") is insulting to families who do manage to make it through a lifetime together.

Lack of commitment, lack of passion and dedication seem to characterize so many of Merser's friends that her conclusion -- that there is no single definition of adulthood, that we remain part child, part adult, and that the mixture makes us creative grown-ups -- seems a cop-out. Merser's narrow focus ignores her generation's heterogeneity and cheapens the efforts of men and women who earn their adulthood the old-fashioned way: doing the hard work of accepting and maintaining their commitments to people, causes and ideas they love, even when it would be easier not to.

The reviewer is editor in chief of Parents magazine.