While his friends scramble for jobs flipping burgers or bagging groceries this summer, 18-year-old Mike Everest will be working as a trader in the fantasy Web world of Entropia Universe, buying and selling virtual animal skins and weapons. His goods exist only online, but his earnings are real. In the past four years, he's made $35,000.

Mr. Everest, of Durango, Colo., is among a new breed of young entrepreneurs seeking their fortune online in imaginary worlds. As the pool of traditional summer jobs shrinks, tech-savvy young...