Players between 18-21 y.o. have the same gaining ability.
But what is changing is the balance in their skills.
When you start training a 18 y.o. defender, normally should start his training with defending (white) skills.
The same next season, but the third season, his defending/white have increased so much so he doesn't earn the same as before two seasons .
Even if his gen.quality is , let's say 95%, his def. skills are 130-140% so he becomes slow trainer to that.
Here an example
This is Gullit, an AMR-MR I bought previous season and added also MC position but I want to use him more for attack so I increased his attacking skills.
I train Gullit with attacking drills and he got 12 skills for 5 sets.
Now I train Gullit with defending/MP skills, where he has low %.
He got 21 skills for 5 sets.
So it's your decision which path you 're gonna take.
Making him a "semi-mutant" (ok, not this player specific because he has too many white skills) but spending lot of greens for development ?
Or continue training with less greens for more balance skills but higher quality ?
If you aim for asso platinum, I guess it's the first choice.
But I suggest to wait and see first the new TE2019.
Maybe we will have new features.