It doesn't have to favour older players if it's done over a shorter amount of time.
Perhaps, instead of a salary increase, a "team chemistry" increase for players who play on the same team for x number of weeks (x to be discussed). When the player joins the team, he has a range of -4 to -2 on his stats (similar to bad GS), then after ever week, he grows slightly until he becomes a random figure between +7 to +9 on his stats.
Maybe it doesn't have to be so pronounced.. Just a thought.
Opinions?