At my old school Jr. BB were black belts with the lettering and such in gold. We had from 1st to 3rd deg Jr. BB's These kids then continues to train in Jr. BB classes, typically learning various weapons forms. Eventually at a certain age they moved up to the adult classes at which point they wore black belts with a colored stripe through it. They would test with adult requirements now and when they promoted they would get a new belt with the black on the outside and current adult color as a stripe through the middle. Kind of like a normal belt with a black stripe through it, this was the reverse.
For my two cents on the original topic, personally in terms of the whole age thing, I earned my BB at 17. I think I was mature enough for it and to understand the significance of it. Though I do agree that the younger kids who haven't even hit puberty yet typically aren't ready for an adult BB, I do think that it should be dependent upon maturity as much as age. And at 15 or 16 if someone has the skill and maturity for the BB I don't think they should be denied just because of their age. I've met some 15 or 16 yr olds that would surpass some 30 yr olds in terms of their maturity and understanding about life.