> If there is a subject where that is the case, I do not know what it would be. Once they're past the age of 7 or 8, most or all children are capable of what they would be at older ages. I don't think, for instance, that an 8 yr old learning to read is slower at it than one learning that for the first time at age 14.

From personal experience: I first encountered differential equations in highschool, while preparing for physics Olympiad. I could solve basic equations, but I didn't understand what I was doing and for example the damped oscillator equation was mistifying. Fast forward to university, in the first diffential equation class, everything made perfect sense and I found it super easy. There was probably at least two years between the two stories.