I remember hearing from people around the time I started lifting (14) that too much heavy weights (ie: machine leg presses) when you're still growing can fuck up your growth platelets and cause you to stop growing (from a bone structure perspective). Don't flame the shit out of me, I don't put much belief in 'stuff people heard'.
I grew to six feet and my original workout plan was from an old ass Weider book, which had a surprising amount of compound exercises compared to a modern bodybuilding program. Still, I don't see the point of loading the kid up with squats and deadlifts too early on.
Seems like I always throw this one out, but CrossFit could be ideal for a kid younger than 14-16, since you're not necessarily looking to get them huge or develop their 1RM's. CF isn't as retarded as a lot of people here seem to think, and it fits the goals of certain people well... particularly people who want to develop their general fitness and minimize fat. One group that fall into that category, I'd say, are kids and youths.
Please don't crucify me, I still think a kid around the age of 14 can go into a bodybuilding style of working out. I certainly did.