They just don't mix. Period.
I would humbly argue that anyone under 18 can't *really* handle a Black Belt.
Yes, there are exceptions, as always, and those should be treated on a case-by-case basis. But never for those under 16. That's where I would draw the line.
Our school has a "junior black belt" program supposedly in place. The *theory* is that you can test for a junior BB at 12, if you are ready. It's never happened, and I doubt it ever will. We allow younger kids to be assistant student instructors, but they really can't be teaching adults until they are older. Thus, most of our student/assistant instructors are 16 or older, easily.
When I look back at where I was when I was 16-18, I laugh myself silly. I thought I knew it all, I was headstrong, and had I had a BB, I would have made a serious fool out of myself. I wasn't ready, plain and simple. No slam intended for those in this age range, but life hasn't settled in enough at that point nearly enough. I think you start getting really ready when you are out on your own, making your own way, and running the rat race with everyone else. That's when you get to use your martial arts in the non-physical way the most (dealing with tense work situations adult to adult, discipline in paying bills, etc).
Again, as always, there are exceptions. There will always be.
I always told my son (13 now) that he *could* be our school's youngest BB, but that it would REALLY require a lot of work, both mentally and physically. He was okay with that. I think most kids are, too, if they understand the reasoning behind it.
BBs for kids under 12? *shudder* Ain't natural. Tell me that a kid can understand all the concepts required for a BB, and can explain them fully to someone else, and I will be doubtful. Most of the *adult* BBs I know are STILL trying to get all the concepts down......
Dang.....I rambled again...

....thanks for indulging me...
Peace--