Early in the year I found myself in a room full of entrepreneurs, part of a "boot camp" run by a development and investment group called Y Combinator. These were the creators of 13 new companies, all with dreams of millions of users and many millions of dollars. And all these companies save one had founders in their 20s, including some who were still college undergraduates. This was no coincidence of demographics. This was the year when Silicon Valley developed an obsession with youth that rivals Hollywood's.

The myth of the peachfuzz billionaire has emerged. This new Horatio Alger typically launches his first start-up in middle school, and somewhere between the campus computer-science lab and a move to Palo Alto hacks up a Web site where users provide fun or useful content. The operation grows, "Sorcerer's Apprentice" style, and soon the budding teeny-mogul's beaming mug is on the cover of BusinessWeek (like Digg's Kevin Rose) or the front page of The New York Times (like Slide's Max Levchin). Not just the business plans, but the fashion preferences and dating habits of this cohort are followed voraciously in tech mags and blogs, in some geeky update of Tiger Beat.

The poster boy for this movement is Mark Zuckerberg, the 23-year-old CEO of Facebook, which he created in a Harvard dorm when he was 19. Zuckerberg raised a few eyebrows this spring when he told an audience of would-be moguls to hire young people with technical skills. "Young people are just smarter," he said.

Zuckerberg later apologized for the remarks, explaining: "I hadn't really planned [the speech] more than five minutes before I talked." He says he wanted to tell his audience that their youth was no impediment to fulfilling their goals, not to imply that older folks weren't worth hiring. No matter the intent, though, the speech seemed to fit the Valley's growing cult of youth, if not a movement, based on "Don't Fund Anyone Under 30."

The reality is not that extreme. "There's a perception that when you're young you can swing for the fences," says venture capitalist Fred Wilson, partner in Union Square Ventures. But even though Wilson has invested in one company led by a 21-year-old, most of his founders are older by a decade or more. "I'm also really enjoying working with an entrepreneur in his late 50s," he says. Jim Breyer, a principal of Accel Partners and the guy who funded Facebook, says that although passionate entrepreneurs in their 20s are the ones who create "truly defining companies," he's also had huge deals from founders in their 30s, and is excited about ideas he's seeing from veterans of Internet companies like eBay and Google.

Still, there's the suspicion that the young are capable of seeing opportunities that the older folks can't get, particularly when it comes to inventing stuff for a medium that they grew up with. "I genuinely believe that this generation is fundamentally different," says Union Square's Wilson. "They're the first generation that's had computers before they had sex." Paul Graham, a cofounder of Y Combinator, says that it's also a generation that benefits from the use of cheap programming tools and instant Web distribution that levels the playing field between a dorm-room innovator and a professional at a big company. "It doesn't matter if you're 19 or not—you can do it!" he says.