21 yo is the limit of full training pace.Starting from 22 yo it will decrease. Generally it's recommended to power train them until 21 and then giving only maintenance, but this depend on how long do want to keep your player in your team.
The question of limit of training it depends on you too. Stronger your team is and stronger your opponents are. I reach an equilibrium, but as you can see others don't agree with me. With time you will learn what's the best for you depending on time you have to spend on the game, resources, and results you want to achieve...