One of the issues (in my opinion) with rank in commercial schools in general is that rank has become a marketing tool. It is used as a sort of diploma that, once earned, means that you've 'graduated.' It is very appealing to parents and to people who want everything to be a self contained package with a beginning and an end.I see - a very different tradition of promotion than that to which I am used, in either the Chinese or the Japanese arts.
As far as I have observed, 'adult' grades only start to come when you are passed eighteen years of age (maybe sixteen in some schools these days) and it should take a period of something like five to seven years to attain a third dan.
A third dan teenager is something I would have difficulty taking seriously, no matter how good his technique. But that's ground we've covered in other threads previously ... and I'm not TKD so I shouldn't really act as if I had a right to 'hold forth' on the subject .
Taekwondo suffers the most because TKD schools outnumber those of other arts by a pretty hefty margin.
Daniel