The attributes of about a quarter of the players in your team will be decreasing anyway from being past their best. And don't expect too much noticeable improvement in players in their late 20's.
If the stats of your younger players are going down as well - what is the overall intensity of their training schemes? It should be around the last notch of medium (or a few notches into hard for the pre-season).
If you've already considered both of the above then there's not much more i can suggest. Perhaps all the players in your team reached their peaks and 20