Elizabeth Holmes is the only woman to make the 'America's Richest Entrepreneurs Under 40'. Holmes quit Stanford at age 19 to start blood testing company Theranos. However recently in a setback, the FDA told Holmes that her company was using an unapproved blood collection device. All of the young entrepreneurs in the list have net worths of $400 million or more and 34 made their money in the tech sector. Twenty-one are billionaires and many either created or work for some of the hottest tech companies, including Uber, AirBnB, Fitbit, GitHub, Instacart and Pinterest. The list's youngest member is Palmer Luckey, who was just 21 years old when he sold his virtual reality equipment company, Oculus, to Facebook for $2.3 billion in July 2014. Luckey's net worth is $700 million and is one of half a dozen in the ranks who are still in their 20s, Forbes said.