Here are the latest ratings. You have to go down all the way to #61 (Nigel Short) before you find a player older than 40. It didn’t used to be that way. For instance in 1963 Mikhail Botvinnik was world champion at age 53.
The game is more competitive, more players come from countries where chess is relatively new (China and India will give you young stars, not old stars), and there is great value from training with computers. If you didn’t start training with computers until you were thirty-five years old, you are at a serious disadvantage.
Consistent with these hypotheses, there are also more and more prodigies in chess. Can you think of any other reasons for the falling ages of top chess players? I also see a general principle operating: the more exact a "science" the game becomes, the smaller is the value of accumulated experience relative to sheer skill.