I am against the whole premise of giving a child under 18 a black belt. If you give him a junior black belt, that is fine, but at 18 he should retest. Not to test his or her physical skills, but to see if his mental understanding and emotional bearing have equaled his physical skills. When I got my Master's rank in two of my styles, it had taken me over 25 years of hard, consistent training to get there. Now, to those who say look at Jet Li at age 11 winning a national championship or traveling to the White House. True, but remember that was 8-10 hours a day for years and except for school on the side, all he did without distraction. Even then, I am sure Wu Bin did not have him leading classes.
Especially these days, most kids do not have the patience or inclination to put in the time. I see my own kids, especially my 12 year old, and at that point in life their minds and bodies are beginning to change and truly evolve into what they will become. As a result, there is no way I would learn or allow to teach a kid this age, unless he/she is teaching other kids under supervision ages 5-10. But that is all.