It's likely because most of the founders who apply to YC are in their 20's (and to a lesser extent 30's) because they are in a better position to live on ramen than older founders. Fresh out of college at 22, you are still used to living in cramped living quarters, eating cheap, and staying up late. As you get older, as you get used to full-time work, you get used to a better standard of living add on expenses like cars, mortgage, and possibly a family. I'm sure there are some who can do it, but it's harder to get a whole family to live on ramen while you pursue a startup.