"They're a growth incubator that pushes companies to grow fast over growing well."

Any evidence for this comment? Any evidence that there's even a tradeoff you can make? Any evidence that your hypothesis (which seems to be that "growing well" is better) is supported at all?

The reason (just guessing, not affiliated with YC) that YC feels like growing fast is best is that "growing well" might involve a greater investment before you've reached product-market fit.

I will also add that age-diversity has almost nothing to do with founders. Founders are already slightly older on average. Plenty of companies founded by young founders have their average age go up quite a bit once they have found product-market fit and can hire more people.