But given the "startup = growth" essay, where he says that only 1 company per cycle makes any difference to YC's returns, I can see the logic behind ageism.
Young people (I'm not one of them anymore) just have more unpredictable outcomes. They are the Black Swans.
The Black Swan theory is that the outliers shape our world. And that has been true in tech for sure -- Apple, Google, Microsoft, etc. are all anomalies. These companies grew enormously quickly and were all started by very young and inexperienced men. The two YC success stories that get pointed to -- AirBNB and DropBox -- were also started by the very young and inexperienced.
Older people are perhaps more likely to be successful. But there is less variance in their outcomes. There's the logic in investing -- once you're making money, you're less valuable. Because people know what you're worth. Before you make any money, people can ascribe crazy valuations to you. It will be wrong a lot of the time, but it doesn't matter because the one Black Swan event is what you're looking for. You only have to be right once.