#6 Elon Musk had already sold two companies before his 30th birthday making him a billionaire

Elon Musk is best known for founding SpaceX, and co-founding Tesla Motors and PayPal.  Musk was given his first computer by his father at the age of 10, an IBM XTA, an early personal desktop computer, and he taught himself how to program; by the age of 12 he sold his first commercial software for about $500, a space game called Blastar.

During his 20s, Elon Musk graduated from the University of Pennsylvania's Wharton School of Business's undergraduate program he also received a second degree in Physics.

He then moved to the Silicon Valley area of California to undertake a PhD in Applied Physics and Materials Science at Stanford. He stayed two days before dropping out to start Zip2, which provided online content publishing software for news organisations, with his brother Kimbal Musk. In 1999, Compaq's AltaVista division acquired Zip2 for US$307 million in cash and US$34 million in stock options.

By 28, Musk co-founded X.com, an online financial services and e-mail payment company, in March 1999. X.com later become PayPal and sold it just four years later to eBay for $1.5 billion.

Musk founded his third company, Space Exploration Technologies (SpaceX), in June 2002 of which he is currently the CEO and CTO. SpaceX develops and manufactures space launch vehicles with a focus on advancing the state of rocket technology. The company's first two launch vehicles are the Falcon 1 and Falcon 9 rockets and its first spacecraft is Dragon.

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