Also, you're right - it's hard to find other kids your age to play music with. Be open minded and think about playing with older people. The older, the better. You can learn a whole lot more from a 40 year old guitarist then probably anybody in your school or anyone you might meet in their 20s.
One of the best things I did as a kid about your age was to join the church band. I had to go to church anyway, so I might as well play bass while I was there. I got thrown in with a couple of professional players (the guitarist and the lead singer) and a bunch of amateurs (everyone else). I really learned a heck of alot at a young age about how to play with others, sound good in a group, and know my role or my position as a bass player.
Look around for other opportunities like that... Maybe there's a band at your church that needs people. Even if they have a regular bass player, they could probably use a backup at times. Just walk right up to them and ask. The worst that'll happen is they say "no."