But I disagree with the implied premise that the best environment for a child is to be around kids exactly their biological age. That is a thing introduced by school system, and in fact goes against human nature. Many kids naturally try to follow older kids and learn from them. In a family, younger siblings learn from their older siblings.
The school system disrupts this natural flow of learning, in order to optimize its own processes. And there are some good reasons to do so; this way you can also teach kids things that their older siblings, older friends, and parents don't know. But this is merely one of many possible ways how to learn, it has some advantages and some disadvantages. (Typical disadvantage is problems with discipline, when small kids learn how to behave from other small kids, instead of trying to immitate their more mature siblings or friends.) And skipping grades still allows you to learn the things that your siblings/friends/parents don't know.
tl;dr - some kids don't want to be around kids their age, and prefer somewhat older company
Personally I think this is a failure to apply technology to education.
If there was a proper individualized track in the school then you wouldn't have a situation like this where kids were going to college. They could be kids around other kids and still have their own learning path.