My 13 year old son throws a "curveball", and has since he was 11. It is different from the curveball that I learned to throw in that he holds the ball with the same grip as a two-seam fastball, but slightly lifts his index finger off of the ball.
His motion arm speed and arm slot are the same as the fastball, but the pitch is slower and breaks down and in. The rotation looks the same and it breaks just like the curveball that I was taught. It took him a couple of years to get command, but it is now his "go to" pitch.
I questioned his coach when my son started working on this pitch; he said that it would not put any more stress on his arm than a fastball would. My son has never complained about his arm hurting, and his strength and velocity continue to increase as he grows. I have been told that as long as he stays on top of the ball and doesn't drop down and try to snap it off, it's not going to hurt him.
Am I being naive and risking his future, or can I relax and enjoy watching him bend it in for a strike?