some time ago i maked a thread that suggest different kind of percentage loos, you can guess is it aplied.
However, in short, i was going with this that players ages 18-21 dont loose anything at end of season but cant train max skill power higher than 10% per season, 22-24 loose 10%, 25-28 loose 20% and 29 and above loose 25% that will follow natural grove and development of players and allow us to spend more time to train youth players that will stay in team for longer