Price tends to be the biggest issue for our startup. I'd love to hire some senior programmers that I know can produce high quality solutions for any problem I throw at them. But from a cash flow perspective, we simply can not afford them. And with older programmers having wives/kids/house payments, they (I'm including myself in this boat) tend to be more risk adverse and not as willing to take the pay cut for potential payout later.

I think there is a network effect as well. When I was fresh out of college, everyone in my close network was all the same age. I always prefer to hire someone I've already worked with or someone that was recommended by a friend. So if the start up is founded by fresh college graduates, their network is going to mostly consist of other young developers.