I'll use myself as a strange situation a well. My instructor has given out exactly one black belt to someone under 16, and that was to me. He has promoted only two people to 4th dan, and I'm one. I was also a strange case though, as I started competing against adults when I was 14, because I had grown about as tall as I was going to get (5'8"), and was just putting on muscle at that point haha. I've also studied by butt off as far as theory, philosophy, and history are concerned.
Funny enough, I am on everyone else's side when it comes to rank. I don't like living in Korea, and seeing 7-year-old black belts (first pooms) and 12-year-old 3rd pooms. In a small school, you have the luxury to do things on a case by case basis, not so on an organizational level, so it seems that standards should be higher. Since I grew up in the United States, I have an idea of what kind of work is put into a black belt, but after spending more than two years here, I have started to rework that idea.
Here's why, the average kid in Korea is just like one in the US. They get their black belt, they're finished. The instructors do it to make money, but unlike the US Korea has middle school and high school competitive teams. Highly competitive, basically professional teams. If you have a gifted youngster, he'll be a poom by the time he gets to middle school, which means he can compete for his middle school team. If he does well, he'll get recruited to a high school program. High school programs with a lot of prestige produce gifted Taekwondo practitioners, who are what the US would consider professional Taekwondo players. They are supported by the school, some live on campus, and they rarely go to class. They spend their days training. I'd say, that a player like that, by age 18 has a better grasp on what they're doing with their martial art than a majority of other people doing Taekwondo. I don't want to throw a percentage out there, because it has no meaning. They've been training up to 7 or 8 hours a day for all 6 years of high school and middle school, when you add that to their training as a youth, you've got someone with 7 years of more than casual experience with Taekwondo and someone who has 6 years of Taekwondo training as a full time job. They are likely qualified to be a 4th dan. From there, they go one of two routes, they only go to get a business/physical education degree from a Taekwondo university like Yong-in, or they compete for the university while doing the degree. They try to make national teams, compete in large competitions (if you get a gold medal in one of the bigger international competitions, it cancels your military service woohoo!) or the Olympics, and your competition career is over by the time you're 27 or 28. From there, you open a school. How do you determine where that person's rank is? They train from 5-13 at a more than casual, say 4 days a week level, then 5-6 days a week for the next 14 years. A 28 year old as a 5th dan or 6th dan seems ludicrous in the US, but when you've been a full-time professional for 14 years who trains 7-8 hours a day?