[microbet]

I feel as smart as ever and like I learn things just as quickly as ever. I also run a mile faster than I could when I was 17.

There are a few factors in why old people are perceived to be dumber and why they sometimes actually are.

Most people let their brains (and bodies) atrophy.

There are a lot of important things old people have done in math and science too. Part of the reason breakthroughs are associated with young people is partly because it is more remarkable when someone does something very young (or very old) and it is remembered. It's also partly because when someone with a new remarkable brain gets his first chance to do something remarkable it's going to be when he or she is young and a lot of breakthroughs have to do with a having a new perspective.

Being a super genius at something also has a lot to do with a moderately long (meaning a few years - not like decades) and extremely intense study of something. For a lot of reasons this is just something younger males are much more likely to do than other groups. It probably goes back to our evolutionary history and it's all scheming to get laid, which I guess makes PUA the perfect science.