Re: should there be an age for black belts?
I think maturity has a lot to do with the childs ability to understand what it is that they are doing. Some kids are mature enough to be respectful in their practice, some are still caught in a "ranking game". I believe it is up to the Sensei though to gauge when someone is ready to test for the next rank.
My son and I train at the same dojo. He is 11 yo and has great respect for the art,for the Sensei and for the traditions surrounding training in a dojo on and off the mat. He gets the relationship between Uke and Nage and isn't a firecracker out there on the mat trying to show off how strong he is by "putting the hurt" on someone.
In November there was a rank test for 5th kyu( I was still a white belt beginer and didn't test until January). One of the younger boys( 11yo) tested and pasted the rank. For the next two months we watched him ordering the "lower ranking white belts" around, insisted on sitting in seiza as " his rank called for" and telling others "I am a yellow belt, I know more then you". I actually sat in his place once to see what he would do, he tried to squeeze in to the place between me and the next aikidoka until his father told him to take a spot in the back row. I typically sit in the back row, and still will regardless of what my "rank" is.
In turn though, this child was a great lesson in observation for the rest of us on etiquette. Don't get me wrong though, there were a few times when Sensei spoke with him about his attitude as well.
I am happy to say that the story has a decent ending. Now that the majority of the class is all on the same ranking, this child has been quieter and less demanding of respect.
So I guess my point is... it is a maturity issue with kids. It is unfair to put an age limit on teaching (accept for the growth plate issue) children when there are those out there that are very willing and respectful to learn.