I know, for a fact, that I was smarter when I entered 8th grade, than I was when I graduated high school. In college, I found a science project report I did when I was 12, about enzymatic kinetics and I didn't know what the heck I'd written! Couldn't understand it! Around the same time, I was doing my sister's algebra II and pre-calc homework for her, it was easy! When I got to Algebra II, I needed a tutor to avoid failing.
I think it has something to do with the fact that when we are kids we absorb information better than we do when we are adults. Math was and is, for me, one of those things where if I think about it too hard for too long I am doing myself a disservice and going to confuse myself. I have to just solve the problem.