as players get older they improve slower and as players get closer to their "potential" they improve slower...

How fast a player improves you can read from markedvalue... players of low quality and high markedvalue improves fast; players of high quality and low markedvalue improves slow...

Age also effect markedvalue significantly, so don't just think jounger players improve faster when buying. In fact if 2 players of same quality and markedvalue, the elder player will tend to improve slightly faster.