Continuing my comments. The question is, what would education look like if everyone could learn at the pace appropriate for them (subject to practical constraints). More able students should not only be at a higher level at any given age, but they should also be able to learn faster, so that "achievement gaps" should widen with increasing age. (For example a 25th %ile student may reach the same level at the end of "college" that a 75th %ile student reaches at the end of high school.)
So not only should gifted+ elementary school kids be learning 2 to several years ahead of the norm, but also kids at the 80th %ile could learn at least 20% faster than kids at the 50th %ile, and so be 2 or more years ahead by the end of high school. [Caveat: I'm guessing these numbers based on my experience. They may not be quantitatively quite right, more study would be needed, but they paint the right picture qualitatively.] Ability grouping, all the way from 0th to 100th %ile, is what is needed to educate people to their level (not just gifted education for the top few percent).
Also different people will reach different plateaus depending on ability (assuming equal opportunities to learn according to ability level) where they really can't progress further because the material has become too difficult for them. And there are many reasons for people to not reach even that potential. One upshot is the "age-equivalents", based on the 50th %ile of a given age, are rather low for an academically oriented kid. If your 7yo kid gets an age-equivalent of 23 or 13 on the topic of fractions, then they are certainly very smart, but more, it says that most adults or teenagers aren't that adept in this topic, and a kid doesn't have to be a prodigy to reach the median for that older age. Another thing to watch out for is when your kid is finding it easy to work several years ahead in math, the course might be pretty easy, aimed at the 50th %ile or even much lower, so the material is not so "advanced" after all.
It's important that a gifted kid can move freely through higher grade material. But when that material is aimed at typical (or struggling) students, the kid will need something else. Two things are (1) tough challenging problems, (2) material not normally covered in the standard school sequence.
(I've got more to say again, but I'll submit this so far.)
