Browsing back through the archives, I found this:
http://news.ycombinator.com/item?id=809233
I found the following thread interesting (quoted in part):
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5 points by jacquesm
[To pg] Are you willing to share the number of companies YC has invested in where founders were in their 20s, 30s, 40s, 50s or older ?
7 points by pg
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.
[...other posts and questions...]
10 points by pg
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.
====== END QUOTE
So, pg is essentially saying that 75-80% of YC's investment goes to founders in their 20s. But he also acknowledges that older founders are more likely to succeed.
I can see multiple possible reasons for this strategy...for example, if you are doing social investing, you might assume that older workers are already established and relatively safe, but those just starting out could use a hand. Or perhaps you care more about disruptive companies than successful ones and you feel that, although older founders may be successful more often, they will be disruptive less often. If, however, your goal is simply to make money, then it seems like you should be investing disproportionately in older founders.
So, what's the reasoning for your investment strategy?
As far as I can tell, the age distribution of people we fund is the same as the age distribution of applicants.