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.
Full disclosure: I'm in the range above, planning to start a company, and not trolling for Y-Combinator dollars.