Why do world champions decline with age?

I could I understand if it was a physical sport but why is it the same case in chess? Wouldn't you technically get better with age as in more experience and time to train? Once you reach your 60's or above I guess for some people their mind isn't as sharp but it seems a lot of chess players peak a lot younger than that?

i think that ur brain function peaks in ur 20s

It has much to do with having less stamina at age 50 ( and above ) than you had in your 20s and 30s . 

It's not just stamina. The brain's calculating power goes down with age. It's not actually a big change unless you have Alzheimer's disease, but in fields highly demanding of calculation skills, like chess, any small change is noticeable.

Stamina as in it becomes a lot harder to focus? I would of never guessed really since I'm still young but I would of thought older people become more patient or figet less during games.

Stamina as in lack of energy to fight/maintain concentration after the first 3 or 4 hours of a game . I start getting sleepy especially when the round begins in the evening and runs more than 2 or 3 hours . This is a problem I didnt have in my 20s and 30s , at all . Ofcourse when you tire and/or become sleepy you are more likely to make mistakes/blunders . I often avoid events now in which there is an evening round or I schedule a bye for that round. In the usual US weekend swiss this usually means the 3rd round , usually a saturday evening game ... I simply dont play them anymore . 

Boris Gulko talks about this in his book.  He mentions something about "crystallized" thinking (tactics) declines with age.  While the other part of thinking, which i cant think of what its called right now (strategy) stays with us.  

It depends on the period. The main question arises is "From how many years is the player playing chess?"

Be polite, helpful and nice!

I'm 66 and while I have only taken up the game in earnest the last year and a half I can most definitely tell a difference in my ability to process information and then recall it than when I was younger. Although I exercise, take supplements and try to eat meals which help me mentally along with not drinking, nor smoking the changes are still there and slowly progressing. 

Although this doesn't really address world champions since I never have been, nor never will be one, it does address the aging and decline of the cognitive powers of the brain from a personal perspective. 

Capablanca, Lasker, Kasparov, Tal ( cause of death = illness ), Alekhine, Keres, Petrosian etc. But it's not aging per se that matters.

Oh sorry, I misunderstood this topic.

You are not a chess player anyway.

Don't we all?

All of the above plus susceptibility to many ailments -- some minor, some major.  But all of which sap one's ability to work at full power and survive a grueling and long classical tournament.  Even Karpov at his peak was said to have lost quite a lot of weight in his matches with Korchnoi and Kasparov.  Capablanca of course had severe high blood pressure problems probably stemming from more serious heart problems based on research in the recent biography.  Even if we just talk about less concentration, greater likelihood of fatigue, and more chance of minor aches and pains that can add up to a strong decline.  Vishy tends to do better in critical tournaments like the Candidates but he can't seem to keep up the pace in the average tournament.  Nor does it makes sense for him to go all out in every event.

Finally those who had achieved high peaks when young might simply not have the same drive and the illusion of invincibility that those who are younger with something to prove can possess.