From a narrow physical aspect, pretty much everyone peaks sometime in their 20s. Your brain is a physical organ and it declines along with the rest of your body. Like your eyesight, endurance, strength, etc. It's the inevitable biological aspect of aging.

But from a holistic wider view, you don't necessarily peak in your 20s as your gains in knowledge, experience and wisdom compensates for the decline in pure physical performance.

Basically, you know lots of shortcuts.