My 13U team used the CU in moderation. I think it is on an individual basis. If the kid is well into puberty and is strong, I think it is OK in moderation. We start teaching it in practice, paying particular attention to mechanics. We call the pitches and limit them to 10-15 CU's per game.
http://www.highviewheat.com/index.asp
http://www.kristensfastpitchworld.com/index.asp