What Happens When a Chess Player Gets Old?

     In the National Football League a player survives, on the average, only three years. The careers of top chess players last much longer, but the optimal age for success varies depending on which study you read.  Generally though it is around 35-42. After that, the chess ship begins sailing. 
     Research has shown that recall was more accurate for younger players and the older players got, the worse their recall got. This suggests that younger players have a huge advantage, but research has also shown that the accumulation of knowledge does not stop until the age of 60. So, experience also has its advantages. 
     From what I have seen of some studies, evidence seems to indicate that once you hit thirty a little improvement is possible, you can remain at your playing level in your forties, but after that you're headed for the back side of the rating curve..unless you are a Korchnoi. 
     One study done at Florida State University suggests that the processes necessary to assess basic chess relationships slows with age, even in experts.   We older players just aren't as quick on our mental feet as the kids.  My explanation has always been that my mind is like a hard drive...the information is there, but it's fragmented and there's so much of it, it takes a little longer to access.
     In a report in the Institute of Labor Economics the authors analyzed data on professional chess players and found that the relationship between age and productivity follows an inverted U: productivity at chess increases by close to 20 percent from age 15 to its peak at age 42 and smoothly declines by 11 percent until age 60. This indicates that better skills and longer experience cannot completely offset the decline in numerical and reasoning abilities. I guess that means if you're over 40 you shouldn't waste your money on chess instructional books.

chess.dom has an interesting article HERE. 
You might also want to read Does Mental Productivity Decline with Age? Evidence from Chess Players HERE. 
Age and skill effects on recalling chess positions and selecting the best move HERE