Won't the players mental stats improve and offset the decreases though when they get older? To be honest, the biggest problem I have with players is injuries, they (maybe not the defensive players so much) tend to be slightly weaker and long term injuries at that age have ruined several wonderkids (both in game and in real life).
Messi probably won't be as fast in a few years but remember the 5-0 classico? I could see him easily adapting his game to a more traditional number 10 role (and this could be the type of thing to consider in FM, adapting a role as a player changes).
With Cristiano it seemed more about developing the mental side of his game more than anything, giving him the experience to go with his determination, to link this to FM I tend to find regens who turn up at 18, 19 (or at least start playing first team games then) tend to develop into more well rounded players whereas a player of 15 with great stats for one role, ie poacher, will end up having phenomenal stats for the some parts of that position (finishing, Acceleration) but generally won't develop as well in all areas needed to be the best in the position (although this may be due to injuries and things like that).
Although just to point out that not every player in real life suffers badly with burnout take a look at AC Milan (Maldini, Nesta, Costacurta, Baresi, etc) when the Milan lab was good they could prolong a players career, Maldini was playing from 17 to 40 I think and arguably spent most of that time as one of the best in the World and FM does reflect this with some players who never seem to dip in form (Like Aguero, who in real life has been playing since 15 I think and is still one of the Worlds best in game at 31 (at least in the saves I've had so far).