Before turning 25, Musk dropped out of his PhD program at Stanford to join the dot-com boom and launch his first internet company, Zip2, which provided business directories and maps, Ashlee Vance reports in "Elon Musk: Tesla, SpaceX, and the Quest for a Fantastic Future."

Compaq bought the company for $307 million four years later, and Musk used the money to launch his next startup venture, PayPal.