I will admit that I never heard a serious argument from anyone that did not rely on issues not related to individual student's education. If you have any materials on those, I would love to read some of it.
Montessori advocates for mixed age classes where more advanced help teach the less advanced, as the teaching itself solidifies it in the more advanced student's mind. Not sure if it is valid. But it is something with studies and meta studies that may have been done well or poorly, not just an aesthetic choice. I think Vulcan style training pods driven by AI like khanmigo will end up being better education than the vast majority of students will ever have access to in person. Or consider it the realization of "a young lady's primer" from diamond age.
Yes, that is an issue. There isn't much separation until age 11 or 12 and even then it is usually at the same physical facility. Is there good data on how to handle that? Don't want to make irrevocable decisions too early, seen lots of people good at arithmetic that can't do math and the reverse almost as often. Then is it bad to have socialization absent between different intelligence levels? Most public schools in the us seem to intentionally leave no time for critical thinking or actual history lessons (as opposed to propaganda).
There is separation, at least here in the US, where basically all Montessori schools are private and can choose who to take, so the extreme lows aren't present. The other aspect is kids' abilities are masked by how fast kids grow and change; I'd guess a 20th percentile 4-year-old will be better at things than a 95th percentile kid 365 days younger, so the mixed ages still helps both, because teaching helps someone learn and the younger hasn't learned the thing yet. Where when you get to teens, the more capable student in, say, grade 9 will be able to learn calculus so much faster than a less capable student in grade 12, either you make the former waste most of the class repeating concepts they already solidly understand, or you end up leaving behind the slow learner. You're hurting one, the other, or both by mixing them. But that's doesn't mean they can't both be in art class, or band, or shop together.
Socialization doesn't have to solely occur in an academic classroom.