Why Is It Throwing A Curveball At A Young Age Potentially Problematic?
The root cause of the problem with throwing a curveball (or a slider) at a young age — young meaning less than 16 or 17 years old — is that at that age the growth plate of the Medial Epicondyle (the bump on the inside of your elbow) is not yet closed. Most pitchers, when throwing a curveball (or a slider) tend to twist or supinate their forearm as they release the ball so as to give the ball topspin. The problem is that this keeps the muscles of the elbow from being able to take up some of the load and instead focuses the load on the UCL. Since the UCL attaches to the Medial Epicondyle, and the growth plate of the Medial Epicondyle is still open for pitchers who are younger than 16 or 17, you can end up irritating the Medial Epicondyle or even pulling it off of the Humerus bone at the growth plate. Fastballs, change-ups, and knuckleballs are relatively (but not absolutely) safer pitches for young pitchers to throw because they do not involve the twisting or supinating of the forearm.
