A 20-something Martha Stewart Courtesy Martha Stewart Some people know what they want to do from an early age and focus on it relentlessly.

Others are driven enough to reinvent themselves, changing careers and industries, and continuously push until they find the thing that works.

Billionaire Mark Cuban, for example, faced hardship when he first started, writing in "How To Win At The Sport Of Business" that "when I got to Dallas, I was struggling — sleeping on the floor with six guys in a three-bedroom apartment." On the other hand, Starbucks CEO Howard Schultz was a Xerox salesman dreaming of good coffee.

As a reminder that the path to success is not always linear, we've highlighted what Richard Branson, Arianna Huffington, and 19 other fascinating and successful people were doing at age 25.