Montessori teaching, at least where we were in the 1980's was very big on "critical periods" for learning tasks. There is a sweet spot where a child can learn things with ease, but when the time is past, it's an uphill battle.
The strongest example is foreign languages, where everyone in America has dozens of examples of children who picked up a language they were exposed to without conscious effort, but teenagers are reduced to tears trying to learn German and adults don't even bother to try unless they absolutely have to. This is supposedly true of music, math, drawing, and penmanship as well, though less strongly. But I don't know that the evidence for any of those is that strong. Everyone is exposed to some music and some mathematics just by walking around, even if they aren't taught any at school, so I'm not sure it's a clear measurement. They would learn different aspects of music and math, not none at all, and this might be enough to build the proper brain structures.