That's more likely to happen as the startup matures. In the early days of ramen-fueled late night code-a-thons in a tiny 1 bedroom apartment, you have a much better chance of finding 20-somethings, not older developers with mortgages and kids.
There's a fairly compelling argument to be made for hiring (somewhat) older folks that goes something like this (attribution: cribbed from Donnie Deutsch):
"Hire 30-somethings who haven't done anything yet. They're hungry to take that step to the 'next level' and prove themselves."
He's talking about hiring people with talent of course; but people who haven't - yet - taken that big leap to really go to the next level success wise. I think there's something to that. Speaking as a 36 year old who feels like he hasn't reached "the next level" himself, it rings true to me.