Why is this high value 18 yo training worse than 23yo's?

[Accyrover]

Jesus wept! give the lad a chance.
Probably still wondering wish way to put his jock strap on.

For your level 5* is high, in other words you will not see the best from him until next season when his head shrinks back to normal and he's a 4*.
I'm guessing he is one of if not the highest in quality in your team, bit of a primadonna.
18yo players are for the future, take what you can now, or PT him to scout, he'll only get slower though.

P.S SP and gains from a match have absolutely nothing to do with performances in the game, they are pre programmed before the game starts relative to his gains % in the training bar.

Case in question, my DMC tonight, last four games form 5-5-4-4 in his last match today gained 2 skill points 156% gains and was injured in the 20th minute.
MF player Sherwood 24yo injured; 30th minute in second game, 1 sp and 75% gains, his normal Avg for 90 minutes.