I don't actually know the numbers. We don't keep track. But I know there haven't been any with founders in their 50s, and only 2 or 3 with founders in their 40s or their teens. Most founders are in their 20s or 30s. Completely guessing, I'd say 15-20% have founders in their 30s.
Off the top of my head, I'd say that older founders are more likely to succeed. They don't give up so easily. On the other hand, they also tend to have much higher burn rates, which make their startups easier for circumstances to kill.
Another factor is that older founders will have (relatively) less need of YC: they'll have their own funds, experience and contacts. The age-distribution of YC startup applicants affects the age-distribution of those that are funded, and so doesn't necessarily generalize to all startups.
E.g. Adobe was founded by a couple of guys in their 40's.
