The last time most adults played kickball was in grammar school. But now, nostalgic 20- and 30-somethings across the U.S. are regressing and taking up the old playground game.

"Most people say, 'I haven't played since I was 10 years old,'" said Jimmy Walicek, 30, one of the founders of the World Adult Kickball Association. "The first time they kick that ball, the smile comes to their face and they are hooked."

WAKA, founded in 1998, has 4,000 members and expects its roster to grow to 10,000 next year. There are active leagues in Virginia, California and Washington, D.C., where five friends started the association. Groups are also being formed in at least 10 more states including Florida, Wisconsin and New York.

"A couple of us were thinking of something to do for the weekend and someone mentioned a pickup game of kickball," Walicek said, recounting the birth of WAKA. "In five minutes we went from a pickup game to taking over the city, to taking over the country, to taking over the world."

So what makes otherwise mature adults want to get back to their childhood?

Jennifer Savin, who helped start the San Francisco, Calif.-based league of WAKA, said after moving to the city from Maryland she was looking for a way to meet people and have fun without getting into competitive sports.

"I saw the ad (looking for players) and I was like, 'Is this for real? Like really, kickball?' When I found out it was the sport I played as a little kid, I was really excited," said the 32-year-old teacher. "And I've met a huge group of people to socialize with."

Each team has a sponsor bar where players go to carouse after every game. And while kickball is not explicitly a singles event, many of the participants are young and unattached.

"We have a couple people who are getting married who met through kickball," said Walicek. "And it's a great networking group."

The co-ed teams make it especially easy to get to know people in a situation that's more playful than the usual bar scene, according to Katie Howard, marketing director for the Boston Ski and Sports Club, which introduced a kickball league this year.

"Guys who were smart, who were looking to find a date, signed up for this," she said. "We tend to cater toward young, hard-working post-college people. But it doesn't have a stigma of a singles club."

But don't expect to look cool playing kickball, which is has virtually the same rules as softball — except it's played with a big red rubber ball and no bats.

"No one has played since fifth grade so it's like everything else. Some people are natural, some people aren't," said Walicek. "But everyone can kick a 10-inch ball whether they are athletic or not."

Some say they took up the playground game to avoid the stiff competitiveness of sports like softball or basketball, but things can get ugly on the kickball field.

"It got more competitive than we thought it would," said Howard. "It wasn't like in grade school where they were like, 'no problem.' It was typical sports hothead players."

But some self-proclaimed sporty types said they'd love to join a team — and leave their competitive spirit at the door.

Tim Cleary, an investment manager in New York, said he has friends in D.C. who are proud of their kickball team's losing streak.

"They don't take it too seriously, but never miss a game," said the 27-year-old, who plays soccer and basketball. "I think the thing about kickball that sounds intriguing is it's more of a social construct than an athletic thing. People are there too goof around and go out drinking."

And Scott Letourneau, who is on the same sports team with Cleary in New York, said he'd fit right in playing the game he enjoyed as a kid.

"I'd definitely play," said the 24-year-old. "I have the mind of an 8-year-old, so I think I'd have fun with it."