It takes a long time to raise up the salary to where its best not to keep him unless you are a Div 1 or 2 team.
I'd keep an 18-19 year old rookie in Div 5/4. Then train him his whole career. Especially if he was MVP.
If you are in Div III or better, its harder to get good training for a rookie unless it is during cup and scrimmage games.
Where as in Div IV/V you can train 3 near rookies no problem.
So therefore you should be keeping your high potential rookies. It will pay off via added merchandise in the long run no matter how high his salary gets.