Those faster trainers than 40/80/120 are becoming more rare... I think Ramirez trained at something like that speed when I bought him when he was 18. It was madness because Ramirez was originally bought as a reserve but since he trained so fast and performed well, thats why eventually he won a place in the team.
I feel if your young player doesn't get 1 point per green booster at 5* then don't bother.
*Retired From Top Eleven*