If you're managing your money reasonably well, and you're in your 30s, you probably have $15,000 in the bank in cash. I've never applied to YC because I've always had an order of magnitude more cash than YC typically invests.
People right out of college, or early in their careers who know they want to do a startup, and who haven't had the time to put aside this level of capital are the prime target for YC, and they are also likely to be younger.