Well I'll give my two cents here:
First of all I'm totally AGAINST anyone becoming a black belt before the age of 18. Call me old fashion and far left traditionalist if you may. But being a black belt is a huge responsiblitity and I don't believe anyone under the age of 18 should be given that rank.. (even those under 21)
I know it's mainly the art of tae kwon do that allows this. It's ashame that many of the instructors and grandmasters have allowed tae kwon do to become the laughing stock of Martial arts. I truly believe that we should be able to see the true beauty of tae kwon do and it's awsome power.
But that's my opinon.