I think he answered your question. Burn rate. YC does not give out a lot of money, and older founders probably burn through it too quickly to be successful.

It's kind of like that betting strategy where you double your bet every time you lose. If you have an infinite amount of money, you will always make your money back. If you have a finite amount of money, well... you lose everything very quickly.

Similarly, older founders may do well, but not before the money runs out.

(I had a coworker who was talking about a friends startup. Millions of dollars worth of initial funding for a project that's very similar to something YC invested in. If your $3000 investment fails, it's not a big deal. If your $30,000,000 investment fails, you become very unhappy. Since most startups fail...)