man why you keep asking in french ?
ok
having 18 years old mean nothing if he is under low category of training , i had one 18 years old who was developed as as bad as 30 years old
players have secret skills called "potential" and normally you can see it reflected in player price .. an 18 years old who cost $10m have sure faster training than an 18 years old who cost $4m .. this is your clue .. but remember when you compare , do it for same age players, like 18 vs 18 not 18 vs 20 as 20 will cost less bcz he is older