Your cycle of replacing players is too short. Even past the age of 21 players that you have had since the age of 18 will continue to make gains. Training to 120% during the first week of the season I can keep players until the age of 23/24. Even if these players stop being trained to 120% when they get "older" their white skills will be much higher than anything you can buy from auction.
Keeping lower rated, older players 100-110%, 23/24 year olds will save you resources and still get results.