There really are people who can start a startup when they're 19. Sam Altman, the founder of Loopt, was 19. But for most people the lower bound is about 22. One reason is that most people need to experience what a job is like to be sufficiently motivated to avoid it. Another is that few people are forceful enough at 19.
Full disclosure: I'm in the range above, planning to start a company, and not trolling for Y-Combinator dollars.
The average age of YC founders is about 25.
After 2 years of corporate work, I'm ready to step out on my own as a result of frustration with what other people want me to do. Sometimes it takes the mother bird to force the chick out of the nest before it learns to fly ....