I use the "shotgun" approach, so I've often got a huge amount of youngsters working the ranks.
I've usually got three "decision points" for a youngster:
When he's too old for the U-18s, if I think he's utter naff, he's done.
At the 20-21 range, if he still requires loans down a division and is something like 4th or 5th in line at his position, I'll move him on - especially if I have a younger, brighter player whom he would be competing with.
By age 23, if he hasn't established himself as a regular in my XXII (
I rotate a lot 
), he's unhappy, won't sign a contract, etc, so I try to offload while I can still get compensation for him.