In my system, kids cannot earn a Black Belt, because they are simply not mature enough to understand what a Black Belt really means, nor can they safely spar with the adults. My instructor uses a ranking system where juniors have 3 degrees of purple and then go to red. When they turn thirteen, they move into the adult class and start at adult white belt. Brown and Black belts are reserved for adults with a minimum age of 17 to earn a Black Belt. This means that people who start in the adult class at thirteen will earn their brown belt at about 15. They can then begin teaching as assistant instructors. Running a full class is reserved for 2nd or 3rd level Black Belts who are over 21 in almost every instance. This works well for the school I attend. It may seem harsh to some, but the kids and parents do not resent it. In truth, this allows people who start when they are little to become phenominal Black Belts when they finally reach the age limit, because they have years of expierience. If they were allowed to earn their Black Belt more quickly, they may get bored (or worse cocky) and leave, never reaching their full potential. There may be a few cases of younger people being exeptionally mature, but not enough to justify a policy of allowing kids to earn Black Belts. Those who do resent it, clearly are not mature enough to be a Black Belt, and the few who are mature will understand why kids should not be Black Belts.