Most schools make it so that a black belt means that someone has a decent proficiency in the fundamentals.
The way I see it, if someone has demonstrated such a command of the fundamentals, then the age really doesn't matter, as to whether or not someone should be awarded a black belt. There are many kids who are mentally mature, and can handle the advanced training, and if they just happen to be 10 years old, the so be it.
Where I would certainly disagree, though, is making a 10 year old an official instructor in charge of classes. Leave that for the adults.
I have no objections to using polished 10 year old black belt kids as good examples in class, and I wouldn't hesitate to assign one of them to help other kids by placing them besides, or partnering them with, a struggling student. In doing so, you help the struggling student, as well as making sure that the young black belt keeps the techniques sharp.
If someone wants a "universal black belt ranking," he can test for one with the USA-NKF, or the WKF. The requirements are on the pages, and of course, you cannot test for a rank higher than the one you hold in your practiced system.