there's two issues here. one is the the ideal world scenario, and the other is the real world scenario.
in the ideal world scenario the peak age is probably your 40's. depending on their fitness - if they are very fit and healthy late forties - if they are fat and unhealthy maybe thirties. there is no reason physically and mentally why you won't just keep improving and improving till you get to the point when your body holds you back. personally, i know myself very well and I am 100% sure i will just keep improving and getting better and better. maybe i will plateau, but i will never be at a point where i will 'wish i could play like i did in my youth', except of course when i reach old age.
however as has already been touched upon there is the real world issue and that is added pressure to win games to feed a family - balancing practice time with spending time with family etc etc. all these issues we are familiar with and have been discussed before.
so to conclude, in an ideal world there is no way a player's peak should be in their twenties - if it was then they probably wasted the 25 years or so after that period of time.