Michael.....i see what you are saying, but nowadays most arts, especially the ones we are talking about are 'do' arts. A path to self improvement. 'Jutsu' arts are the pure battlefield stuff.....and to be honest i've never known a young black belt in a jutsu art, probably (my best guess) because of the maturity needed to be able to understand and implement concepts of a jutsu art. Take jujutsu, if a class does have younger kids in (a lot dont let them till maybe 14-16), then they certainly wont be doing the really nasty 'battlefield' stuff that the adults would learn. Blimey some of the locks are absolutely devastating and children whose bones are still forming, no matter how good they are, cannot (imho) demonstrate such as amount of skill and control so that they wouldn't hurt a training partner or even themselves.Also, Banzi Joe. Correct me if I am wrong, but isnt a martial art in its purest form a battlefield or war driven art, something not sportified. Similar sized and weighted opponents will not always be pre-determined and if the child gets into more than he can handle simply thinking that he is a Black belt in x,y or z and is either beaten or hurt, it will do nothing but dent that child's confidance, something which is not good at a formative age.
So, my point being, that 'do' arts are really a path for self improvement, hence the existence of many younger dan grades as they progress on their path. Also, i have never known a young dan grade who has thougt himself invincible due to his grade. I have found that it brings out a maturity and pride in them that could help avoid them turning into the yobs half of todays kids are.
Most of what i have said above is opinion, but the some, im pretty darned sure, is either fact or just common sense.
Its just peoples perception of what a black belt 'should be' that raises this interesting question. I have no problem whatsoever with seeing 8 year olds achieve junior dan grades after probably training hard for 3 years or so and proving they can acheive the task sets by their sensei's and not people who are ignorant of what it truly means to work hard for your rewards.
btw, that last line was in NO WAY directed at anyone. Its been a good topic, opinions vary and i value all the ones i read. I'm sure others do too.