If the curve ball is thrown correctly, it is only slightly harder on the arm than throwing a fastball. So from this standpoint, you don't need to wait until puberty to throw the curve. However, the complete answer is that is must be thrown correctly and the number thrown should be limited (unlike what you might see if you were watching the Little League World Series :x ).
BUT, not all coaches know how to teach the curve ball correctly. Furthermore, it's next to impossible for the human eye to see what the throwing arm is doing in realtime to determine if a pitcher is, in fact, throwing the curve correctly. Therefore, there is a danger that a pitcher could be throwing it incorrectly. This is probably the best reason for waiting until puberty.
In any case, I feel kids will attempt to throw them on their own so it's better to teach them to throw it correctly.