t's fair to say that the singer-songwriter who calls herself Gwendolyn never thought her band, the Goodtime Gang, would appeal to anyone over the age of, say, 7. A typical performance includes covers of the preschool standards "Bingo" and "The Itsy Bitsy Spider" and original compositions that tackle topics like human anatomy, the importance of sharing and bugs.

So it was with some surprise that Gwendolyn, who is 28 and performs in a Raggedy Ann dress, cartoonish pigtails and knee-high socks, found herself one recent evening in a packed Los Angeles nightclub performing for a crowd of fans whose idea of a stiff drink extends beyond undiluted o.j. Many in the audience sat cross-legged on the floor, cocktails perched on bobbing knees. Some sang along.

The performance was part of a bill that began with an elaborate puppet show and ended with an appearance by the Trachtenburg Family Slideshow Players, a "conceptual art rock band" from New York, which includes a 9-year-old girl on drums. For Gwendolyn, who has no children of her own but who says her songs for children are inspired by "a 4-year-old kid inside me," performing children's music for an audience of grown-ups was more than just a hipster lark — it was liberating.

"All the inner children of these adults are suddenly speaking up and saying, `Hey wait — what about us?' " she said. " `It's our turn to have some fun.' "

From childless fans of kiddie music to the grown-up readers of "Harry Potter," inner children are having fun all over. Whether they are buying cars marketed to consumers half their age, dressing in baby-doll fashions or bonding over games like Twister and kickball, a new breed of quasi adult is co-opting the culture of children as never before. Most have busy lives with adult responsibilities, respectable jobs and children of their own. They are not stunted adolescents. They are something else: grown-ups who cultivate juvenile tastes in products and entertainment. Call them rejuveniles.

Celebrated by market researchers and fretted over by social scientists, rejuveniles come in all ages but are mostly a product of the urban upper classes (free time and disposable income being essential in their lifestyle). Evidence of their presence is widespread. According to Nielsen Media research, more adults 18 to 49 watch the Cartoon Network than watch CNN. More than 35 million people have caught up with long-lost school pals on the Web site Classmates.com. ("There's something about signing on to Classmates.com that makes you feel 16 again," the "60 Minutes II" correspondent Vicki Mabrey reported.) Fuzzy pajamas with attached feet come in adult sizes at Target, along with Scoobie Doo underpants. The average age of video game players is now 29, up from 18 in 1990, according to the Entertainment Software Association. Hello Kitty's cartoon face graces toasters. Sea Monkeys come in an executive set.

And a big hit on Broadway this summer is "Avenue Q," which stars googly-eyed puppets grappling with career disappointment, maxed credit cards and failed relationships. Part of the show's pleasure — besides the puppet sex — is the "rediscovery of the real attachments we had to creatures like this as children," said Jeff Whitty, the librettist. "It awakens the kid in us."

No single word has emerged to describe the phenomenon, but a few phrases in the marketing lexicon describe some of its aspects. The San Francisco advertising firm Odiorne Wilde Narraway & Partners calls the resurgence of retro brands among 18- to 34-year-olds "Peterpandemonium." Toymakers now take aim at"kidults," defined by the Italian company Kidult Games as "adults who take care of their kid inside." Researchers at the MacArthur Foundation are studying "adultolescents," those 20- and 30-somethings who live at home and still depend on their parents for emotional and financial support.

While some marketers court rejuveniles directly — "Who knew you and your daughter would have the same best friend?" asked an advertisement for a revived line of Strawberry Shortcake dolls — others speak to the rejuvenile soul by simply selling to kids. The Honda Element, the Tonkalike mini-truck introduced by the company as a "combination dorm room/base camp for active young buyers," has been marketed mostly at extreme sports and surfing events, said Andy Boyd, a spokesman for the American Honda Motor Company. But the average age of Element drivers, Mr. Boyd said, is 40. "That's exactly what we anticipated," he said. "It's a new definition of the family buyer — someone who doesn't want to give up their individual character even though they're getting older."

While there is nothing new about adults reveling in kiddie culture — Shirley Temple, Roald Dahl and Pee Wee Herman all had plenty of adult fans — market researchers say an especially strong wave of childishness began about two years ago. Milk and cookies, macaroni and cheese and meatloaf began appearing on the menus of highchair-free restaurants. Puma, Converse and Keds sneakers leapt from the schoolyard set to the fashion-conscious crowd. And then there is Harry Potter, whose cross-generational popularity prompted the British publisher Bloomsbury to release an edition of the books with so-called grown-up covers. (Adult-friendly kid titles are listed in Booklist, the trade magazine, under "Crossovers: Children's Books for Adults.")

"We're seeing this phenomenon worldwide," said Debra Joester, president of an independent licensing company that handles Care Bears, one of the lines of discontinued toys and merchandise recently reintroduced in part because of pent-up demand from grown-ups. (Other resurrected brands include He-Man and the Masters of the Universe, My Little Pony and Rainbow Brite.) A 2001 market research study by American Greetings, the creator of Strawberry Shortcake and Care Bears, showed that "purchase interest" was identical among women who wanted to buy a doll for their child and those who simply wanted to rekindle a love affair of their own.

"This consumer wants Care Bears in their life," Ms. Joester said. "And not just to share with their children."

In part, researchers say rejuveniles are simply seeking comfort in jittery times. Who better than a character like SpongeBob Squarepants to relieve free-floating anxiety? According to Nickelodeon, a full 26 percent of SpongeBob's regular audience is over 18.

Some social scientists, however, see signs of a deeply troubling trend. That so many adults expend so much time and energy pursuing the thrills of youth just proves how significantly "adulthood has lost its appeal," said Frank Furendi, a professor of sociology at the University of Kent at Canterbury in England. "Adulthood has got nothing attractive about it anymore. That's actually quite sad."

Mr. Furendi began researching what he calls "the self-conscious cultivation of immaturity" after spotting college students watching "Teletubbies" in a university bar. The scene stuck in his mind, and he came to think of it as representative of a wave of infantilism sweeping Britain and beyond. What is happening, Mr. Furendi maintained, is a natural if extreme response to a media culture that equates being old with being square and being young with being relevant. "Today, the way you demonstrate your worth is the extent to which you still go to rock concerts, you're still groovy, you're still a player," he said.

But many of those who fit the profile best — grown-ups who wear Sesame Street T-shirts or skin arthritic knees on their motor scooters — insist they are not simply obstinate Peter Pans or connoisseurs of kitsch. Many describe a nearly frantic compulsion to remain playful, flexible and fun in the face of realities like fixed-rate mortgages or lawn care. Mitch Anthony, president of a branding and design firm in Northampton, Mass., is a full-fledged adult: he has children, a closet full of suits and a picket fence that cost $10,000. But as he approaches his 50th birthday, he sees "absolutely no reason to give up doing what I loved as a kid," he said. "I still bike. I still love to hang out with my friends and talk about sex. I still play in a rock 'n' roll band. Why would I want to stop doing any of that?"

Rejuveniles reserve their deepest respect for adults who manage both to take care of business and to make time for play. The skateboarding mogul Tony Hawk is 35 and the father of three, but he is a hero of rejuveniles everywhere for staying in sync with the "12.5-year-old suburban male" who represents his core audience, said Pat Hawk, his sister and business manager. "He's not trying to live in a fantasy child world," she said. "He skates for a living, and he gets to travel in a private jet. How cool is that?"

Still, it is one thing for a 12.5-year-old to idolize a guy like Hawk. It is another for his dad to pine for a life of nose grinds and front-side kickflips — as many do. (Ms. Hawk describes her brother's adult fan base as rabid.) Bryan Page, a professor of anthropology and the chairman of the department at the University of Miami, said: "Play has historically been about recreation or preparing children to move into adult roles. That whole dynamic has now been reversed — play has become the primary purpose and value in many adult lives. It now borders on the sacred. From a historical standpoint, that's entirely backward."

Many rejuveniles, however, reject the notion that their enthusiasms are childish in the first place. "I like Chipmunks records because they're funny, period," said Jacob Austen, 34, a Chicago writer and authority on music by Alvin and the Chipmunks, part of a genre of children's music fans affectionately call rodent rock. Mr. Austen, who also produces a children's dance program on Chicago public-access television, says the best entertainment for kids is universal.

Ironically, most actual kids could not care less about much of the stuff that enchants rejuveniles. Take "The Langley Schools Music Project: Innocence and Despair," a CD of Canadian schoolchildren that has been praised by the likes of David Bowie and John Zorn, who called it nothing less than "music that touches the heart in a way no other music ever has." Irving Chusid, the record's producer, said that what adults find haunting, kids find utterly mundane.

Mr. Austen said he understands that distinction all too well. While friends and family have come to regard his love of old kids' records as charming or sweet, children are less forgiving. "I get the most censure from little kids, definitely," he said. "I'll be playing a Chipmunk record in my car, and if a kid hears it, they get seriously weirded out."