Have you discovered an upper bound? The commonly held belief is that people over X (where 30 < X 35?) can't start startups because they have kids and a mortgage, or they just aren't in-tune with what's hip. Curiously, that excludes people who have "been there and seen that", the best current example being AJAX. Also, those who have taken a few more trips around the Sun also may recognize niches that 19-25 years would never see. My impression is that Y-Combinator is an attractor for the younger entrepreneur.
Full disclosure: I'm in the range above, planning to start a company, and not trolling for Y-Combinator dollars.
Harder to say about the upper bound. The factors you mention may play a role, especially kids. I also think need for money is a factor. Most people who could make money have by 40, and once you have enough, you're less willing to endure the pain of starting a startup.
The average age of YC founders is about 25.
I couldn't agree with you more. It isn't until you really experience what it is like to work for someone else before you realize what it's like to work for someone else.
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 ....
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.