Dude, that's impossible to say. It's about performance, not time spent. And grasping of the art is not purely a function of time, but also of effort, average frequency of training per week, consistency of training, and natural abilities.
Originally Posted by Black 6
Furthermore, a black belt may have a different meaning accross arts. For example, BBs in BJJ and Judo are not necesarily parallel or equivalent. Also, any art that on average awards a BB in 3 years or less, is bullshit IMO.
Going back to the BBs, to attain a BB in BJJ it takes an average of 6 to 8 years whereas in Judo it may be 4 to 5 (I'm pulling those number out of my ass, just regurguitating what I've read before... somewhere on books and on the interweb.) This is assuming constant, consistent training and plenty of competitions. Some people are just naturally talented, and can progress at a much faster rate a-la BJ Penn, but the vast majority of mortals do not fall into that category.
And that's for adults. Going back to the kid, I'd bet a kid that starts at the age of eight would eventually get his blue belt at the age of 16, and if he is awesome, will get his BB at 18. So that means, it will have taken him 10+ years to get his BB. Independently of that, such a kid would have been a monster on the mat by the time he was 14.
-- the opinions expressed in this post are just coming out of the ass of El Mighty El Macho, scourge of chickens, plight of goaths, corruptor of women, and in no way reflect the thinking of the bullshido comunity, nor it may necessarily have the legal endorsement of the Eternally Undead God Helio, The Holy Kano and The Mighty Kimura, may I ride their nuts forever.