(FORTUNE Magazine) – Who knew that internet millionaires had such staying power? In this, our sixth compendium of youthful success, we discovered that though the Net bubble has long since burst, a third of this year's lineup of nouveaux riches are actually not so nouveaux--they're holdovers from our inaugural list in 1999. Tech entrepreneurs, by contrast, have seen their numbers dwindle by half. Their replacements: the likes of Britney Spears, Jennifer Lopez, Julia Roberts, and the Olsen twins. Yes, ever more of the young rich are entertainers, athletes--and women.

This year we returned to our original criteria--ranking the 40 richest--after last year's inclusion of innovators, influencers, and entrepreneurs along with the ten richest. To compile this year's list (all members younger than 40 as of our Sept. 20 issue date), we sorted through thousands of candidates, excluding those who inherited their millions or earned most of their wealth outside the U.S. We pored over proxies, insider-trading records, and other SEC documents. Thomson Financial helped us to identify the richest and to research insider trades. Navigator Capital Group provided estimates of the worth of private companies. For celebrities and athletes we turned to industry reports and grilled attorneys, agents, and producers. Nielsen SoundScan tracked music and video sales. Our final tallies include salaries and revenues from endorsements and box office sales, as well as the value of investments, exercisable options, and private-company holdings. We adjusted for everything from taxes and assumed spending to agents' fees. But with the Google guys near the top after their IPO, it's a reminder that Net geeks can still make billions before turning 40. At least with them, unlike the actresses, we don't have to envy their glamour too.

Behind the numbers In the top ten

How hard is it to stay on top? For Michael Dell, not at all. But for the others, remaining at the peak is no mean feat (see chart). Some were dropped because they hit 40 (Amazon's Jeff Bezos, Gateway's Ted Waitt, Broadcom's Henry Nicholas, RealNetwork's Rob Glaser, and Michael Jordan), others (Monte Zweben of Blue Martini, Scott Blum of Buy.com, and Vinny Smith of Quest Software) because their wealth no longer made the cut. With the Google men only 31, they may be the next two with Dell-like staying power.

Dell's farewell

Among the downsides of turning 40: losing eligibility for our 40 Under 40. That means the perpetual king, Michael Dell, will abdicate, so to speak, after this year. His wealth, of course, will live on--and not just because of his company's stellar stock. In 1998, Dell began diversifying, launching MSD Capital as a private investment fund with less than half-a-billion dollars. Six years and lots of Dell stock sales later, those assets have bloomed to $10 billion. MSD's recent stock picks have been down-to-earth: tiny Tyler Technologies, which provides IT services to local governments (up 45% in a year); Darling International, which collects grease from restaurants (up 66%); plus restaurant chains IHOP and Steak N Shake. And when MSD dove into real estate this year, buying a Four Seasons Resort on Maui for a reported $280 million, Dell's money managers took out a mortgage, like anybody else. There's a reason he stays rich. --A.H.