From what I've seen, 21 is young, in the sense that there are a lot of people who aren't ready yet at that age. But by 25 the kind of people who make good founders are ready.
I suspect the phenomenon is more social than biological. The problem with the 21 year olds is that college tends to preserve some childish illusions about how the world works. So the reason 24-25 is the sweet spot is that it's far enough away from college for those illusions to have worn off.
Somehow academia/research was nice earlier and I had some luck getting tolerable positions, etc. But it's been tough keeping the wolves at bay lately. The average academic department is a real viper's pit. And crossing chasm from research to a product that people are willing to buy is not easy. Perhaps not even possible in my case...
It's OK. I never really aspired to be a big-time industrialist. I was always more interested in having an impact in a hard area. Lately I have come to the conclusion that the best thing for me to do would be to get back to publishing... If you try to role out a product as an academic, your publication rate fall off and the vipers come out and bite you (Usually in the form of giving you lots of bad teaching assignments and a variety of psychological warfare techniques).
But it's hard to convince oneself that most of what academics write has any effect. I suppose I could focus more on databases, but natural language interfaces seem to be where there are still fundamental problems. But our publications in that area are usually so dense that only fellow academics bother to read them...
It's not easy to change the world I suppose :-).
However, I do have the problem of finding like minded individuals at my age that are willing to take the risk of joining a start-up becuase they are used to the comforts of having a "secure" job. They tend to see doing a start-up as a bit of hobby, but this doesn't bother - I wouldn't want them to work with me anyway because I expect the same commitment to the project as I have.
Anyway, at 28, I am still going to apply for YC. Power to the old giffers!
The message is that anyone over 30, not to mention 40, need not apply. Of course this does not really bother me, but it makes the whole high tech start up thing sound like "Americas next top model." LOL!
"We want 'em young and fresh!" And yes 25 is young. Very young.
That said, I think PG comes off as a thoughtful and honorable person - which I already knew. I noticed some quibbling with the interviewer about if "creating wealth" was the right phrase. It's not important, but it is interesting to see that quibbling never ends no matter how successful one is.