It seems more apparent by the day. The paint-balling CEOs, skateboard-riding grandmothers, and video-gameplaying, cartoon-watching fathers form a new generation of American adults who refuse to grow up, who embrace silliness and surround themselves with the paraphernalia of youth.

Part pop sociology, part marketing manual, and part lifestyle manifesto, Christopher Noxon’s “Rejuvenile” (Crown, 288 pages, $23.95) makes the insightful case that a sea change has occurred in the way American adulthood is lived.”Rejuvenile,”Mr.Noxon writes, “describes people who cultivate tastes and mind-sets traditionally associated with those younger than themselves.” With this broad definition, he tries to understand his subjects – most of them in early middle age – not by the markers of economic and social achievement (or underachievement) that typically dominate discussions of Generation X mores, but by their immature habits. Mr. Noxon’s proto-rejuveniles have a career, a home, and maybe even children, yet still prefer dodge ball to tennis, Monopoly to bridge, and feel unmoved by social pressure to give up childish pleasures simply because they’re, well, childish.

And why should they? The elasticity of modern American adolescence has occasioned more than its share of handwringing, but Mr. Noxon argues that it may not be such a bad thing after all. He suggests that adulthood as we think of it today is less a biological imperative than a byproduct of the Industrial Revolution, child labor laws, and the ironically named “play movement” of the 19th century, all of which – along with the need for an increasingly literate, specialized work force – necessitated the creation of a strictly bounded social holding pattern called childhood. “This shouldn’t suggest that people in olden times didn’t distinguish between kids and grownups,” he writes.

It’s a mistake, though, to confuse maturity with adulthood. The maturity celebrated in traditional rites of passage … is not the same thing as the idea of adulthood hatched a century ago by a coterie of Victorian clergymen and society ladies. Maturity is old. ‘Adulthood’ is new.

This may be a stretch, but Mr. Noxon is definitely onto something and believes that the days when play was exclusively “the work of childhood” are coming to an end.

He identifies the market forces responsible for the increasingly blurred lines between childrens’ entertainments and the rest of the culture and points out that “the only movie in the top ten all-time grossers that even resembles a movie for grown-ups is ‘Ti tanic.'” He goes on to trace the lucrative “family entertainment” genre back to the 1904 stage success of J.M. Barrie’s “Peter Pan.” He examines Disney’s highly successful “Fairy Tale Weddings” (some 1,000 performed a year), and tells us that when the makers of the Honda Element marketed the boxy SUV as a “combination Dorm Room/Base Camp for active young buyers,” it was actually “part of Honda’s plan to reach buyers who hadn’t seen the inside of a dorm room since the Reagan administration.” The average buyer for the Element turned out to be 40 years old, right on target.

Though something of a haphazard reporter and perhaps overly dependent on secondary sources, Mr. Noxon is at his best when defending the rejuvenile lifestyle against critics who see in Generation X and its successors a cohort of self-indulgent wastrels failing to take on the mantles of adult responsibility and productivity. Where these voices, which he dubs “commentators from the Harrumphing Codger School of Adulthood,” complain about later and later marriages,he points to falling divorce rates and attributes them to people knowing themselves more fully when they take the leap; where they fret over young people waiting longer to have children, he sees a generation of better, more involved parents who “work hard, worry plenty and spend their share of sleepless nights mulling over household finances – but unlike the parents [of the 1950s] they don’t confuse these acts with parenting.” Mr. Noxon even challenges the widespread stigma attached to adult children still living with their parents, pointing out that, from a global perspective, our emphasis on single living before marriage is a cultural particularity – and a relatively recent one, at that.

But the book’s anecdotal feel and dearth of compelling statistics leaves one straining to get a sense of just how widespread a phenomenon this actually is. The absence of a discussion of irony is a glaring omission; despite Mr. Noxon’s own patent sincerity, much rejuvenalia practically drips with the stuff – often in ways that don’t exactly jell with his sunny theses about the adult value of childlike openness and wonder.

Where he really falls down on the job, however, is in ducking the hard work of finding a way to usefully understand the various segments of his amorphous rejuvenile demographic. By lumping adult kickball leagues (team-picking rules amended to avoid reprising childhood humiliations, of course) together with, say, the distinctly unchildlike obsessions of the grown-up Disney fanatics known as “Disnoids,” Mr. Noxon fails to bring his manifesto to fruition.

Still, the best parts of this book are entertaining and even inspiring, and I couldn’t help but be struck by the simple truth of the observation, quoted in “Rejuvenile” from the theorist Brian Sutton-Smith, which sums up the rejuvenile attitude at its best: “The opposite of play isn’t work. It’s depression.”

Mr. Garin last wrote for these pages about Martha Stewart. His history of the cruise ship industry, “Devils on the Deep Blue Sea,” was published last year by Penguin.