Oh, grow up!”

That’s a mantra that has been chanted for generations, one that suggests that settling into pressure-packed adulthood and leaving behind the joys of childhood is a grail to be pursued.

But now, what adulthood means is being re-examined, as detailed by author Christopher Noxon in the book “Rejuvenile: Kickball, Cartoons, Cupcakes, and the Reinvention of the American Grown-Up” (Crown, 2006).

In his book, Noxon points to examples of this phenomenon, including:

. Half of the 200,000 people who visit Disney World every day are adults without children.

. Movie studios turn to comic-book characters in an attempt to widen the age appeal of their films.

. The average age of video game players is 29 and rising.

. Thirty-eight percent of single adults between 20 and 34 still live with their parents.

. The Cartoon Network boasts bigger overall ratings among viewers 18-34 than CNN.

. Adults are putting off marriage and having children later than ever before.

While Noxon notes that some critics see these developments as a sign of the end of Western civilization, he disagrees.

“I don’t think it’s quite so dire,” he said in a phone interview from his California home. “The leisure pursuits that adults allow themselves to do can lead to a happier, more enriching adulthood.”

Noxon admits that some hard-core rejuveniles are living in their parents’ basement while collecting action figures. But most that he wrote about have found a balance in their lives.

“You still have to be responsible,” he said. “But you don’t have to give up everything you loved growing up.”

Noxon, a freelance writer whose work has appeared in The New York Times, Los Angeles Magazine and Salon, admits to living a rejuvenile life even before he began researching the book.

He had been looking for a book-length topic to begin while serving as the primary caregiver for his two young children.

“My life was watching SpongeBob, eating Popsicles and going to the park, and I was having some of the best times of my life,” he recalled. “I was already a childlike adult. I met my wife (“Weeds” creator Jenji Kohan) while playing kickball. I noticed a similar phenomenon among people my own age. Then I went out and found people of all ages that felt that way. That showed me that something dramatic had happened to the concept of adulthood.”

Noxon points out that these changes have come in an increasingly unstable world.

“The world is scary and uncertain, and this stuff is not,” he said. “The qualities that we are able to reawaken are things that are suited to help us navigate in it. The world is so up for grabs, and the people who are prospering are figuring out how to adapt, quickly. And those who are best at adapting are children.”

Following such rejuvenile forefathers as Walt Disney, J.M. Barrie, Theodore Roosevelt and Lewis Carroll, among those Noxon profiles in “Rejuvenile” are a fiftyish Ohio couple who jet off to Disney World once a month to hook up with other “Disnoids,” a twentysomething legal aide from Kansas City who promotes “All-Ages Tag,” an L.A. singer-songwriter who creates rock songs for children and a San Francisco CEO who has created a museum devoted to his favorite boyhood amusement park.

As the number of adults looking back fondly grows, business has responded, with products geared to invoke nostalgia across age groups. These include the VW Beetle, cell phones with video games and pop-song ringtones and clothing with popular cartoon characters from the past.

“I worry a lot about the influences of the media here,” Noxon said. “There are obviously sophisticated market forces at work here. But you also have adults who have woken up to the realization that they don’t have to have the Ozzie-and-Harriet adulthood. The pressures to conform aren’t there as much anymore. And all that marketing still has to resonate with people to be effective.”

Noxon argues that “playalong parents,” who join in their children’s playtime, make better parents.

“Having kids today is no longer seen as a mandate to become a role model of authority and seriousness,” he writes. “Talk to parents today and you hear more about connection than obligation, more about availability than leadership, more about participation than provision. The most idealistic playalong parents have a deep-seated commitment to appreciating children for who they are – not just as incomplete adults in constant need of instruction and improvement but as people in their own right who might even teach us a thing or two.”

Still, Noxon preaches temperance to potential rejuveniles.

“It’s easy to slip into a childish brattiness or impatience,” he said. “They need to take stock and figure out what’s useful and enriching to them as adults.”