Interesting paper on the demographics of American entrepreneurs:
...a larger proportion of tech founders are middle-aged, well-educated in business or technical disciplines, with degrees from a wide assortment of schools. Twice as many U.S.-born tech entrepreneurs start ventures in their fifties as do those in their early immigrant tech founders, U.S.-born engineering and technology company founders tend to be well-educated. There are, however, significant differences in the types of degrees these entrepreneurs obtain and the time they take to start a company after they graduate. They also tend to be more mobile and are much older than is commonly believed.
See also this summary.