I'm 32 and my co-founder is the 33 year old mentioned by Paul. We're a YC this year. I didn't particularly feel like age would have played any role in acceptance (though I believe we have the most traditional business model and most traditional product in the bunch, and had Trevor and Robert not been interested it may have triggered a "no" from Paul). There are three PhD candidate founders who are probably much closer to 30 than 20, as well.
Anyway, I didn't notice any particular agenda for picking younger founders...younger folks are the ones that showed up (I met all, or nearly all, of the folks who got to the demo stage in Boston..including the majority that didn't get picked).
