Re: should there be an age for black belts?
Just my 2 cts:
I think 18 is a good age for a general rule for shodan requirement, but more as a rule of thumb.
While I do not think that most 18 year old are mature enough for receiving shodan, even if they are technically perfect, there could be a few younger girls and boys that are. So you better leave space for exceptions. Remind that shodan is not a teaching licence per se.
On the other hand it sounds a little bit strange, if you allow young peaople driving 300 hp vehicules right after a week-end seminar, if they wear guns without any education or test, but you do not allow them just to wear a black belt on the mat (outside they can wear whatever they want).