I, too, am a young developer (15 when I started contributing to open source).
I can speak from experience when I say that no one cares. No one will ever ask your your age, and unless you are immature, no one will have a reason to.
Just remember that other people will usually know best, as you're still young. Learning should be your priority. Of course, there's no reason you can't learn and contribute at the same time!
Anyhow, I suggest starting with tiny, tightly-knit open source projects. Small communities are more likely to teach you, and take you under their wing when you mess up, as opposed to silently rejecting a patch.
To start out in open source, just lurk on the mailing list for a while and see where it takes you. If someone asks for something small (an easy patch, obvious bugfix, etc.), volunteer and write it up yourself. It will help you get acquainted with the code base. Once you've got the feel for it, feel free to grab TODOs from the issue tracker and try and implement them.
One last thing - try to get code reviews as much as possible. Not only will it prevent embarrassment when a "rookie mistake" sneaks into a release (and someone will, inevitably, git blame you), it's also a wonderful learning experience when people suggest better ways of accomplishing tasks. It's so exciting to find out something that took you 200 LOC and 4 hours of your time can be accomplished in two API calls. Most of your programming problems have been solved by someone else already.