The "real" issue is assuming preteens and young teens have the same hormonal response to weight lifting. Some "experts" worry that the natural boost of testosterone associated with a heavy session will confuse the body's initiation of (or duration of) puberty.
I bet the Russians and Chinese are sitting on a goldmine of info.
Also, I'm sure it was mentioned... But I don't know at what point a child's bones become stronger than their ligaments.