Given the recent episode where Zumaya blew his arm out and one of the kid that I coached in little league is currently having arm trouble from pitching(bad mechanics), I thought that it's a good time for me to address the parents or parents to be of their child in baseball(or in other sports in general) on teaching your child to do the proper techniques or mechanics early on so they can stand a better chance of staying healthy. This is true for pitcher because there are a lot of young people going into Tommy John surgery which is mind boggling because it shouldn't have come down to this type of procedure.
I am not a parent, I'm still fresh out of graduating from college. I cannot stress enough that teach your child(or children) the proper mechanics of throwing a baseball. It's very important for long term health and it would greatly enchance their game as well. I would much rather have my child suffer for a while while doing proper mechanics rather than dominating with terrible mechanics. Over the long run, your child will thank you.
If your child is a pitcher or is about to be a pitcher or you're a little league coach, I would not recommend on letting your child throw curveball until they can start shaving or 16 year old. It put too much stress on the developing bones of the elbow and it would lead to arm problems down the road. I would have them on concentrating on throwing fastball and change up.
IMO, overusing is not the reason why there are so many arm problems. It's pitchers using bad mechanics is the cause for arm problems. Overusing magnified the bad mechanics. If you have great mechanics like Roy Oswalt, Tim Lincecum, you shouldn't have any arm problems and still throw hard into the 30s.
Finally, long toss is a great arm excerise/drill for any baseball players to do in practice especially in warming up(except for those who are pitching on that day). It builds arm strength and stretches out arm muscles without having to throw hard. Your arm bounce back quicker than usual(of course it helps that I lifted weights, but I wouldn't recommend your child doing it until they reach high school).