Somewhere around 150 or so. If you are looking to play professionally look to taking vitamins and supplements and exercising.
The mind can very sharp up until a very old age. Especially if you use it like a chess player! Hint! Hint! I know people say you can't play as well as you get older but I don't believe it. I'm 57 and continuing to improve from 1630.
A chessmaster told me he once tried and tried to become a chessmaster and failed ... Until he quit trying and began to enjoy the game. He was a chessmaster in 6 months.
Based on personal experience, I'd say 65 is too late to get "serious" about chess and expect to get "good"
. Luckily you can still have fun even if you never get past patzer
.