I guess it will help you, because it gives you some face time with the YC founders. The reason I would debut my site here is if it fails it will happen quickly and early on. That means that I won't be wasting time on something that's not worth it.
Can't hurt. If YC thinks YC founders are smart, and YC Founders think your app is cool/well-done, it's likely that YC will give a more serious look to what you've built.
(disclosure: I've never applied to YC. I think I'm too old!)
Too old? You're never too old. I think people don't do themselves any favors by disqualifying themselves, in ANY arena. If someone else wants to, they can, but that's their problem -- always be confident you bring a lot to the table and others may miss out.
Humans are not infallible machines. It's like playing chess against someone: if you play you always have a chance, and your chances are only ever 0% if you resign.
How old are you?
Usually not much one way or the other.
What do you know?
Why the downmods? Isn't it reasonable to expect that anyone reading here would recognize "What do you know" as a joke?