Especially for defenders that do not grow quality so much from the games gain quality, max age for me is 24.
Same for attackers, but as they can develop easier through season, if they are great performers it can go till 26-27.
Unfortunately I do not enough in-game resources, to maintain a solid 7* star team (which is my season target) and keep star old players.
At least last 5 season I sell them at the right time/at the right price so I have room to experiment and spend some tokens trying to find the ideal replacement.