In today’s fast-moving digital world, CEOs of companies are becoming younger and younger. Some common traits many successful CEOs under 40 share are their early entrepreneurial spirits. They had good ideas young and built on them. They started their companies young, and many of them began running them as publicly traded entities.  Here are just a few success stories:

• Kevin Plank, 37: Invented Under Armour his senior year of college. Tired of sweaty T-shirts after football practice, he developed the prototype for his moisture wicking Polyester/Lycra Blend T-shirt and grew from their ever since.

• Michael G. Rubin, 37: Started a Ski Store when he was 12, dropped out of college to start KPR Sports, specializing in athletic shoes and apparel. He grew the company, took it public and in 2002 renamed it GSI Commerce. Today it has a market capitalization of $1.3 billion and builds and runs websites for retailers.

• Michael Chasen, 38: Started building computer programs for local businesses for $25 an hour while in high school, he eventually dropped out of law school to start Blackboard, the online learning company utilized by many academic institutions nationwide, and went public on the NASDAQ exchange in 2004. Today it has a market cap of $1.2 billion.