I'm totally against curve balls at this age, I would say until the age of roughly 15-16 they shouldn't throw them. How much of a benefit is it to throw a curve ball at 11 years old? The majority of any breaking action is just plain old gravity. The problems of long term health you can cause is huge. A player at that age has so much growing to do with tendons, ligaments, and growth plates to be putting undo stress on them. I coach a 18 u national team and still there are to many players that can't locate their fastball and throw a good change up. I think younger players should be more concerned with learning to hit spots and changes their fast ball speeds. They have plenty of time to throw a curve ball, but the damage they can start at such a early age sometimes can't be irreversible. Learn to hit spots and change speeds rather then adding a curve or trick pitch to the repertoire.