Some people know what they want to do from an early age and focus on it relentlessly.

Others reinvent themselves, changing careers and industries until they find something that works.

Billionaire Mark Cuban struggled 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."

As a reminder that the path to success is not always linear, we've highlighted what Richard Branson, Arianna Huffington, and 17 other fascinating and successful people were doing at age 25.