Now we seem more on the same page. What you have said is largely in line with what I was thinking, with a few minor clarifications.
At least we're now moving in a positive direction.
As I said in one post when deciding the requirements for each belt there can be a whole range of criteria that a teacher could add to a system if they so chose. These things go beyond just physical skill and knowledge. For instance if the designer of a system was to decide black belts should be able to teach, then the instructor would only pass a student able to teach. Or an instructor could decide that a black belt is mature, and would only grade someone to black belt when they are mature (that is, rather than when they're supposed to be mature, e.g 16/18/21 etc.).
In many schools, assisting with classes, is a pre-req. for getting your BB. I started helping when I was a brown belt, and I will freely admit, I was nervous. The assistance went from doing just the warmups, to that as well as the basics, ie: punching, kicking, bag work, etc., to taking a group of lower ranked students and working on their material, to one day, when my inst. told me I was doing it all. Of course, he was on the sidelines to offer aid if needed, but things went smooth. My point of saying all of this, is that taking command of a class, being able to teach in an effective manner, and at the same time, keep things moving, so as to not lose the interest of the students, is IMO, not an easy task. If the person heading the class isn't mature enough, chances are, they will fail at the above mentioned things. This is why I was looking at it from an age point of view. Can a 14yo be mature? There are some that are, and some that are not. As I said in my other post, if it could be a feat to expect a 10, 12, 14, etc yo old to be mature, either way we look at it, theres a good chance they may have to wait until they're older anyways, to reach that maturity level.
Further holding a child back from grading too early, and a big part of the reason I would not start teaching younger children the main syllabus, is the inability for their body to practice certain techniques safely, thus limiting what syllabus could be taught to them, but I also take into account that people develop at different ages and thus it is easier to look at each child's physical and emotional development rather than going "10. 10 is the minimum age, no exceptions."
From what I've seen, many schools do limit what is taught. Example...if your typical 4-5yr old class, the required material is cut in half. So at some point, prior to the child reaching BB level, the other half is going to have to be learned. This, I assume, is where the Jr. BB comes in. They get something they can call a BB, even though its not an 'official' BB, they continue to grow and mature, help with classes, and learn the other stuff that will be required for the full BB.
On the other hand, if we look at some highschool and college courses, there are pre-reqs to certain classes. Ex: Completing Spanish 1 before moving onto Spanish 2, even if the student is breezing thru S1 material. They need to meet certain requirements before moving on, so if we apply that to the arts, its really the same thing.
Finally, I am not against holding people back from their grading, but when the only reason is their age, I think that's just silly.
I think this is where I still may be a bit confused. I think we both agreed that it would be a superhuman feat for some kids, to be a certain degree, one reason due to maturity. So, if thats the case, then in essance, they're still going to have to wait until they're older, so if they have to wait, I'm looking at it, as if there is no difference than if we said the child would have to wait. Sure, I will agree that there will be a handfull that will be the exception.
As I said in the beginning when I spoke of teaching and how it takes alot to be able to command a class for a full hour or longer. There has been more than one time, when I've had to 'prove' myself during a teaching session. Now, this isn't necessarily meaning that it was physically, althoug there were times when people insisted that something would not work, and the only way to get that point home, was to have them come up, really take a swing at me, and have me show them. Now, I didn't send them home with a black eye or busted lip, but the message was clear.

Is the 13yo 3rd degre BB going to be able to do that? Physically being able to apply the tech. is just 1 part, the other is to also be able to verbally explain the fine points. Again, some may, but some may not, and this was the reason, for me, why I felt that some may look at that young age and high rank, as laughable.