Yann Golanski wrote:
Remember that due to physionomy, under 16s should not use wrist locks. It can permanently damage said wrist.
I started training when I was 12. My wrists are fine. So are my brother's.
Black belts should be handed out when the students are ready. At my dojo back in Seattle that meant we had to be training in the adult classes, which my brother and I both started doing as soon as we had the size and ukemi for it. There's also a junior black belt system in place at that dojo that was developed after I went away to college to accomodate a couple very young and talented girls who were too small to train at the adult level but too good to be left in the brown belt ranks (the kids got colored belts; when we moved into the adult ranks we had to take an adult kyu test and switch to the white ones - I was actually kinda proud the day I traded my brown belt in for a white one).
As far as a teeenage shodan losing focus after their test, I've seen adults do the same. This usually happens among those who just wanted the belt, and this attitude is under no circumstances limited to children.