Training for fun and traing for rank are differant. IMO no kid under 16 should have a Black Belt. Thats just me agree or not. Their is way to much mental and emotional developemt involved in training. On the flip side I have saw many talented kid between the ages of 8 and 12. Some younger... But do they understand the mental part of karate in most cases I would say no. They are just athletes... I think the sooner you get kids involved in any sport or activey that teaches morals, balance, motor skills you are going to make that child better off. However giving a child a rank it is not mentally ready for can be very dangerous IMO. One note about people in other countries they don't test for belts like we do. You get one belt and thats it. Training is a life long process not somehting you take up for a few years and grow out of.