My biggest issue with YC is that the deal isn't very good. Giving up 7-10% for $500k is pretty crappy for any company that has a decent shot at becoming anything good. That is why I have only ever applied to YC when my company ideas were very speculative. If you already have a network and a decent platform to communicate with people, as well as the motivation to go at it for yourself, you're giving up a huge chunk of the company for a pittance.
If you already have the ability to go talk to a VC and an idea/product/company that is good enough to make it into YC, you don't need YC for your company. If you can sell YC on investing in you, you can sell your product.
However, it does make sense to do YC right out of college to get the networking benefits and the credibility signal. That's why so many 22 year olds do it. It sacrifices your stake in your first company in exchange for helping you as an individual later in your career.