A question I was thinking about this week: Why do some GM's drop in peak performance? It's common to see players have a golden age of performance where they are near unbeatable. But unlike physical sports, GM's don't exactly suffer from physical deterioration do they?
The mind is just as physical as anything else.
It's a deeply complex question, but generally speaking most forms of cognition -- including processing speed, working memory, and fluid intelligence -- decline with age beginning in your twenties:
https://www.quora.com/Intelligence-and-Genetics-Can-does-IQ-...