Re: should there be an age for black belts?
in my dojo we start kids classess at 4, this is not so much about aikido but about discipline and fitness, after they have done the "little dragon" syllabus they will move into a junior regime at the age of 8, here they start on a specific syllabus designed for children to which they are able to achieve a junior dan grade, we also provide a form of cadets for the older teenagers that aren't ready for adult classes, if any of these children attain junior shodan they will continue in their relevent age group classes wearing a black belt but not a hakama, until they are ready to join the adults, upon joining the adults they start a program which guides them through the adult syllabus giving them coloured stripes on their black belt for the relevant kyu grade achieved on the adult syllabus, then when ready will grade for adult dan grade, which has to be older as we still use live tanto for our dan grades. so in my opinion if you have the right syllabus in place there shouldn't be a question of age for certain tests.
as for the part of higher grades training with lower grades, i think you have to pretty up yourself to believe you can't learn anything from a low kyu grade, and perhaps you should looking at yourself and asking what you are doing training with this kind of view. if you open yourself to your aikido you will learn as much from training with 7th kyu as you will from 1st kyu. and surely a 1st kyu training for dan grade should need to learn the humble side of aikido and train with lower grades, rather than selfishly training for oneself rather than for the group. i feel that humility is an important lesson to be learnt by all.