Check their terms. Unless they have an actual restriction against founders under the age of 18, why not apply? I honestly wouldn't even worry about whether or not they'll consider you.
Applying for just about any incubator program can be a great way to round out your idea, make sure you've covered your bases, and to just learn about the process in general.
Don't get your hopes up--credentials and track record can be just as important, if not more than, the idea itself. Just don't think you're walking away empty-handed even if you're not selected.
The biggest roadblock I believe you'd face is that you are legally a minor (under US law, I suppose other countries might vary by a few years). This would limit your ability to enter into binding contracts, funding arrangements, etc.
The most recent TechStars Boulder class included 2 co-foudners ages 16 and 17.
If you have the right talent, drive, team, and possibly idea, then age should not be a problem.
Bare in mind, that if you do get funding from Y Combinator you'll need to adhere to their rules - including
If we invest in you, your group is expected to move to the Bay Area for January through March 2012. (You can of course leave afterward if you want, but it's a good place for a startup to be.)