If you throw a curve the traditional way before you are 14 & 1/2 you risk tendonitis in the elbow tendons according to Dr. Joe Thomas an excellent Orthopeadic Surgeon in Cincinnati. It has to do with the heel of your hand leading as you accerlate through your motion and snap your wirst. It is even worse with sliders. Chris O' knows all the corect medical terms I am sure.
I am a huge fan of the breaking ball. The curve and the slider were resposibile for a lot of my pitching success in baseball. This being said, I can not watch the Little World Series where I see kids throwing hook after hook at ages & 11-12. For the love of Christ, they are not little adults! I almost get ill watching it; this not the way pitching is supposed to be. I leave the room.
The first book I ever read on pitching gives excellent advice, If you want you own best pitch develope your fastball for : Location , Movement, & Velocity. Me being a breakingball guy, I still threw more fastballs than all the other pitches combined. Look at Bronson Arroyo who throws 3 or 4 different breakingballs, still his fastball is his number one pitch.