For a short period of time, yes. For an extended period, no. And maybe I was a little hyperbolic. Maybe a 5 year old, for several hours a day. One with the same type of personality I had as a kid (which was if I like something, I do it nonstop). And/or one with parents who are heavily into martial arts.
Remember, my situation is hypothetical, not anecdotal. And there are kids that train hard their entire lives from an early age to become professional athletes. There are lots of videos of really young kids (I'm talking in the 4-5 year old range) who are already very competent at martial arts. They don't get to that level by slouching at a young age.
My point is, how much experience does someone have if they train on a normal schedule of 3 classes a week, 2 hours a class, and they practice that much on their own as well. 12 hours a week. After 40 years, they'd have 24,000 hours of practice. Someone who practices with that much dedication for 40 years is probably going to be in 7th or 8th dan territory.
Someone else, who trains 6 hours a day, 6 days a week, would need 14 years to come to that same level of experience. 24,000 hours. Does the person training 12 hours a week not train enough to be worthy of the title? Does the person training 36 hours a week need to be held back?
My original example was 10 hours a day from 3 years old, but this example would work for 6 hours a day even from 20 years old. Now, these are obviously just random numbers. Maybe you think the person should be training for 20 hours a week for 50 years to get 8th dan. Well, it's not hard to do some math on that, either. 50k hours, which someone who trains 40 hours a week could get in half that time...if they started at 10 years old, that's possible by 35. And it's a lot less than the 70 hours/week (10 hours/day) I quoted above, starting at an age more reasonable to your criticisms of my approach.
So how much experience should someone have for 8th dan? If it's possible to have the same amount by age 35, why shouldn't that person have it?