A fast trainer can reduce is value in 25% a 31 years old player Will not lose value.
The reduction in value takes in account the age and the speed of training, every player regained is old market value in 2weeks, só they always increase the value during the season.

In my first season i bought 18 years old 3* fast trainer fir my reserves i sold them with 21, 22 years old, with only the Daily training i gained the double of the price i paid for them.

If you keep the players 2 or 3 seasons you always Will gaine money.