yes I have sold most of my key very good players at the age of 24-25, as when they are at 99% it is better to sell them and getting some tokens (people tend to make good offers with players with good stats) than train him and spend x2 or x3 the green packs.

Of course this has some risk as you dispose a key player and you never know if you will find an ideal replacement.

But this is how the game engine works so we should adapt accordingly although it is a bit unrealistic comparing with real football