the older they get the slower they train.
18-22 they have no problem to keep up with your team but after that they train much slower and when they hit 26 they are done.
You can buy a cheap 28 year old 5 star player, great for replacing a injured players, but next season he will be 4 star, season after that 3 star.
After 26 you need to spend at least 200 tokens per season on personal trainer just to keep the player at lvl 5.