You're a little late observing this. I've seen this being done for more than 25 years. And in that time the sky hasn't fallen or much else except society has gone to blazes in general (just a gentle jest).
Training groups are what they are, they're not all equal.
Rank is what it is, and it's not all equal.
I've been training young people for over 25 years now, and on the average it takes them 7 to 9 years to become a black belt, and by then they're no longer young.
Is it controversial, not really. I disagree with your definition that black belt means expert. I hardly consider new black belts experts in any system I've seen. I think a better definition is they're no longer beginners and are now students.
But the issue behind young people that I have isn't age (actually I've seen some incredible 12 and 13 year olds, including one 13 year old young woman to took first place in National Isshinryu competition in weapons, against all adult men in the division).
The main issue I see is they really can't choose to be a student. They're under their parents guidance. Their parents make them get to class (and drive them) and support them. They haven't made their own choices to be a student and enter deeper training, and at 13 very few can do that.
But it's relative. If we are in dislike of the pactice, it's so universal in some groups at this point, nobody anywhere is really going to take notice.
Big world isn't it.
bushi no te isshinryu
offering free instruction for 30 years