Date: August 2nd, 2015 7:09 PM
Author: Concupiscible slate sound barrier
raw mental processing speed peaks in early 20's.
accumulated knowledge continues growing for a while, but the slowdown in speed starts taking a bigger bite by the late 30's.
by the later 50's, knowledge accumulation compensates less and less for the natural mental slowdown.
during the 60's, neurological changes associated with outright senility begin to manifest.
by 70, you're largely done here.
(http://www.autoadmit.com/thread.php?thread_id=2952166&forum_id=2#28464177)