The Curveball
Teaching or allowing young pitchers to throw curveballs is controversial. Reasons that are given for not throwing a curveball are that it will hurt a young pitcher's arm (particularly his elbow) and that it hinders the development of a good fastball. Reasons for teaching the curveball include it will not hurt the pitcher's arm if thrown correctly and that a curve ball is needed by the time they get to high school. I think that all of these reasons are legitimate and, in general, would like to see a pitcher wait until he is at least 14 before developing this pitch. And, given different types of arms and stages of development, 14 is a fairly arbitrary age.
As a general rule, the younger the pitcher, the fewer curveballs he should throw and the slower he should throw them.
If a younger pitcher has any soreness in his elbow (or any part of the arm) he should give up throwing curveballs for at least a couple of weeks (and shouldn't pitch for a while as well).
Curveballs should be thrown with just enough speed to get them to the plate during the first year or two of throwing this pitch. A hard curve or slider increases the risk of injury.
If the curveball is thrown flat (lots of horizontal spin), it is not as effective as a curveball that breaks down and sideways and it increases the risk of injury. Young players often think the curveball needs a lot of sideways spin. To achieve this, they drop their elbow below their shoulder when throwing it which is not only bad mechanically but puts too much pressure on their elbow. Getting kids to stop throwing with a dropped elbow is one good reason to teach them how to throw a curve ball correctly.
Mechanically, the curveball should be thrown like the fastball with the exception that the hand pulls down in front of the body instead of extending out toward the target and the wrist turns downward instead of snapping forward. In addition (although this varies to some extent with the pitcher), the path of the ball is a little closer to the head, the stride is a little shorter and the ball may be held a little further back in the hand.
Gripping the ball also has variations. Some of the best curveballs are thrown with the seam running along the inside of the thumb and the fingers perpendicular across two seams. The most popular grip is to put the fingers fairly close together running along a single seam.
If a young pitcher has the potential to throw 90 m.p.h. in high school, or the coach cannot be trusted to keep curves to a minimum (in a game or practice), or a pitcher tends to get a sore arm (especially at the elbow), or a pitcher has not spent a good deal of time working on a change-up, or the pitcher is overused (or throws to much on his own), I would recommend that he NOT start throwing a curveball.