Players only be able to train at maximum speed between 18-21 years old after that they decrease a but between 22-25, then a lot between 26-29 and after that the training rythm is just to slow.

You have already noticed that players with same age and quality and diferent prices, that happens because the cheapers 18 years oldWill train like a 26 years old players, taking a lot to improve.

The players with the higher price Will train very fast, being able to gain a lot of SP average of 2-3/day only with dayli training.

Beware being a fast trainer, dosent mean he Will perform better that a slow trainer, he just Will train faster, being able to be in the team more seasons.