Try him for a few matches.
Does he score freekicks regularly?
If yes, would be worth having him just for that.
Else, you can try keeping him for a few seasons as a rotation player and then sell.

PS: I was following exactly what you do....buy 18 year olds...sell them at 21-23.
On seeing the success of many "older" players, I have changed this and have a good squad of players now.
Majority of my team is above 23 now and they perform pretty well.