http://www.youtube.com/watch?v=LdumfxLGkSY&feature=related Slow motion capture of pro women beach volleyball.
You begin to lose the ability to "see" and/or maintain in your mind several variations of the next 10-15-20 moves. A younger more agile mind will be able to "see" and maintain many more variations and therefore will have an advantage over someone who is not able to "see" and/or maintain as many variations in their mind. Joe.
I have a theory but it is my own and is highly speculative. a) Chess is a game of war. But it is also sport, science and art. I suspect one key reason is the reduced testosterone levels in older men. It is imperative that you be in top physical conditioning to play chess. Most people do not realize this, and maintaining a high level of testosterone is probably key to high level of performance intellectually. Older men have reduced levels of testosterone. b) Chess is the only one of three categories where you can be classified a prodigy. The other two are music and mathematics. I don't think this is an accident. What do these three have in common with each other? I believe that their commonality lies in the strength and health of the Corpus Callosum http://en.wikipedia.org/wiki/Corpus_callosum Chess, music and mathematics are probably the most intense experience known to man that engages both sides of the brain. I believe that it is this decay that causes dramatic drops in chess playing strength in older players. It is interesting that in the general population, the CC is thicker in women than in men, and yet men dominate chess and mathematics, but they hardly dominate music. It is also interesting that Monotremes and marsupials do not have a corpus callosum. As we understand aging better, and specially brain functioning, I believe that we will see world chess champions in their fifties and maybe sixties. But one thing is for sure, these people will be in top physical shape, and while externally they will look older (until we learn how to make skin supple even at advanced age), internally they will have the minds and bodies of much younger people.