Its child dependent. ACS makes some good points.
It really comes down to the rate at which someone learns. If a child is going through a year's worth of learning in a month ( I could read most textbooks in a night), then how much value will be that education be if they spend 90% to 95% of their time twiddling their thumbs?
I agree that rate of learning is a big part of this, but it also has a lot to do with where a person's interests lie. DS certainly is academically gifted with test scores to prove it; he could function at a college level based on his ACT and SAT scores. But he is not nearly as driven to do the book-learning that Austin was.
The things that he most craves learning are not textbook subjects. He wants to know how clubs are run; he wants to devise the best strategy for pulling ahead at a cross country meet; he loves trying to figure out why the teachers present the material the way they do, what language they use, why they assign the projects they do; he wants to know why certain kids hang out together; he wants to figure out the best way to disarm the bully. These are the things he comes home bubbling with excitement about. He wants to use his wit and social skills. He is subject accelerated in math and enjoys it and does fine in it. But he says he feels like he doesn't fit in socially with the older kids so just being with them one period a day is all he want.
So I guess I would say that rate of learning is part of it, but also it depends a lot on preferred mode of learning (DS is verbal and kinesthetic), introvert/extrovert, drive to move ahead academically, and interest in "non-academic" subjects.
