Why Do Top GMs Decline?

What are the causes of decline in top GMs?  

Chess is not a physical sport, where there is an overwhelming reliance on physical attributes, such as jumping ability, speed, agility, coordination, etc., in which the decline of many of those attributes is rather rapid after the age of 33 or so.  Instead, chess relies primarily on the mind (and secondarily on other things like physical fitness), which can presumably continue to be in top shape until....,say, one's 50's (correct me if I'm wrong)???

So why is it that top GMs seem to decline prior to that (correct me if I'm wrong here) and it's the younger players that rise to dominate the game of chess?  

Is it because of a better facility with technology?  Or perhaps is it better physical fitness (although it's not an excuse, because you can keep yourself in good shape even into your 50's if you work out), which gives them perhaps a small, but sufficient edge to win close games, due to stamina?  Or is it maybe even the slightly better mental functioning of younger players (i.e., they think more quickly perhaps)?

Yet, what of the experience of the older players?  Wouldn't that help in some situations too as a counter-veiling force? 

They get bored of playing chess. I am so bored of my jon. But everyone saus that i have to stay there