A 18 y.o. player is a new player in the game. So, where did he come from ?
A. He is an academy player, from a team above you. If he is a 0T academy player,then that's why he is a slow trainer.
After you bought him, you can check (in news letter) info about his previous team.
B. If info is unavailable, means he is a nordgen ( a player from a bot team/ a non existing team).
Nordgens are the faster trainers - if they are the most expensive. The problem in higher levels like yours is that we can't see the difference in prices, like we can see it in lower levels.
If he is a 1st class nordgen, must hit a +100 in 3 bars practice. If he is 2nd class, maybe a +70, +80. (read previous posts).
In that case, I keep the player and test him in the games. If he is performing good, I keep him with the idea that I 'll spend some more green to keep him in high quality for some years. The same for a 3rd class nordgen.
C. A player A. or B. whom some other team bought him and train him or use him. He wasn't satisfied from the results and sold him.
So, what was the info from the previous team ?
What was the results of training ?