I was thinking the same way earlier, not worth keeping players over 21, at least a little bit longer. That also came from the standart football manager logic I know - maximize your money, so developing young players is great for this, I thought.
Then I realized we are all oligarchs here lol, also I observed (probably not obvious for all, but I see this tendence), that too young players often perform bad in key games. And finally the white skills thing, which I started to follow at some point, so it became harder to say good-bye to my players. So now I am running my first 15-seasons-cycle