I guess it comes down the individual. You're right, certain players do reach a peak in their early 20's, and can keep their speed for a fair while, but others can start out at 'top' speed so to speak, suffer injuries, or just age, and they lose it. Michael Owen is a good example. I'm pretty sure Walcott too will some day begin to lose his speed and acceleration, probably through an injury, and then his main attributes will leave him pretty limited as a player. But I think these two are good examples of players who probably started out at their quickest when teenagers. Depends on how differing individual's bodies age and progress I suppose.
Not true, there aren't that big of differences in peak speed regarding the age of a person and it can be clearly seen from observing top sprinters and their best times.
From observing the performances of top sprinters from the past 50 years it is well established that peak age for speed is around 24-28ish, with a few exceptions stretching it to 29 or 30 and 23 to 22, but generally from late early twenties to late twenties.
However that principle can not be applied to PES because it is a soccer game and players here don't really have a speed training regime as sprinters do, add that to the fact that some players get lazy and slack off and gain some weight, some get injured which screws with their progress and you have a different thing alltogether, so in my opinion it would be very hard for KONAMI to realistically add losses of stats based on so many factors as real life has.
For example lets say Walcott slacks off and gains some weight and his top speed suffers, how can PES replicate that? Randomly? Wouldn't really work well.
Even in their early 20's though I think it's a constant physical battle to keep at the peak of their physical fitness, or else they can lose that vital half a yard of acceleration, and it's something I'd personally like to see in future PES titles.
Again, training in general is always very hard but as long as you do and stay in shape it shouldn't be a problem at all, but soccer players have so many variables its hard to replicate this to PES.
IMO regarding physical stats they should just go with what generally applies and that is:
Agility will start to fade at the start ( does not mean u have peak agility at certain age and can not improve it, but the peak u can have starts falling from the moment ur born ) so generally assuming everyone is at peak it should generally not raise and start falling slowly fairly early.
Acceleration and Reactions ( Not PES stat reaction, but real reactions alltho pes doesn't rly have a clear stat for simply reactions of players, but maybe it will in the future ) are next to go, u should not be able to improve them much from the early age and it will start to fade in mid twenties.
Top Speed you can improve a bit more than the first 3 things but only if you put alot of effort in it and it starts to fade very late twenties to very early 30ies.
Jump a bit more than Top Speed and fades a bit later, early 30ies.
Strenght/Balance this one is tough, on one hand peak physical strenght is in the 30ies or even early 40ies but on the other hand its hard to replicate that to PES because if you get more buff you get more weight and hence ur other attributes should generally suffer more ( agility etc.. ) but in PES as you raise your balance you don't really gain weight so not really sure how id use this, maybe some of you have any ideas?
Stamina also you start low and can improve it throughout your career alltho slower as you get older or better at it.
Injury tolerance is what I would like to get worse as you age because the more old you are the more prone you are to injuries specially after 30ies, but also I would like them to add an 8 stage system like weak foot accuracy for it not just A,B,C.