I did some research after asking this, and the studies' results seem to contradict each other. Apparently, our brains are largest at the age of 14, and we're smartest at about 15, especially in terms of new concept absorption. While that may be true, I once read that a fatty substance called myelin, which apparently facilitates rapid and efficient processing, is most ubiquitous in the brain in your early twenties, and a gradual decrease in this substance is probably responsible for mental decline. But a major longitudinal study on the topic found that mental abilities are largely constant from 20 to 60, and that through training, elderly brains can be significantly revitalized. I theorize that a major confounding factor is the tendency to retire around 60.