Comment by u1hcw9nx

It's the ability to concentrate that starts to go.

Kasparov have talk about this. Older players can play at a world-class level for the first few hours, but their ability to maintain intense concentration declines as the game progresses. Most blunders by older GMs happen in the 5th or 6th hour of play. Older players also can't recover from earlier intense game next morning as well.

According to Kasparov older players get "calculation blackouts" and inability to visualize the board.