The average YC founder is about 25. The range so far has been from 19 to (I think) 33.
"The other reason it's hard to start a company before 23 is that people won't take you seriously. VCs won't trust you, and will try to reduce you to a mascot as a condition of funding. Customers will worry you're going to flake out and leave them stranded. Even you yourself, unless you're very unusual, will feel your age to some degree; you'll find it awkward to be the boss of someone much older than you, and if you're 21, hiring only people younger rather limits your options." - How to start a Startup
To make that situation work I think you need to believe strongly in two things:
1) Being a good leader does not mean being the smartest/most knowledgeable person in your company. It means being able to spot the smartest people and make them work together and learn from each other. That's not as simple as it sounds
2) You MUST believe 110% that age is a completely irrelevant metric when determining the "smartness" of a person. I don't care if someone is 20 years older than I am - if they haven't done a start-up then that puts me ahead of them in respect to running a start up. They may be an infinitely better coder/designer/whatever than I am but not a better startup founder.
The second reason is why I ditched my career as an attorney to be a start up founder. I don't intend on letting my income be determined by the number of years I've been sitting behind a desk.
"We care more about how smart you are than how old you are, and more about the quality of your ideas than whether you have a formal business plan."
Perhaps the question should be: How many startups have you been a part of?
YC founders probably differ from the average founder more in how hackerly they are than their age. We have a strong bias toward companies run by the people actually writing the software.
Experience is important, passion is more important.
Experience without passion is a dead startup.