I don't think the primary driver for any startup applying to YC is cash. The value in YC or Founder Institute or Techstars or any of the other accellerator/incubators is advice, mentorship, learning and connections, and the implied social proof and credentialing provided by acceptance. It's easier to live on ramen noodles in your twenties out of school. Having done gone down that path, the sacrifices to do it with a family are almost impossible. It is rough when your little kid looks up at you when the school lunch account is overdrawn and says "I'm sick of being poor, Dad. When are you going to launch?" So it makes sense for YC to focus limited funds where it can get the best yield. Younger, highly intelligent males of extraordinary ability without mates who are out to prove themselves and build social status and wealth often do. That's why the crime rates are higher at younger ages too.