It's not some arbitrary age that he hits that will make him better. It's about being young and his body still developing physically.
Look at guys like Vander Esch, who have two years on him in age. You just have to give the guy time to develop, if they don't, then you move on from him when his rookie contract is up.
I swear, people on this board just think that if you aren't a HOF player your complete trash. They think that if a rookie doesn't come in a light the world on fire he is a bust. Hello, a lot of guys struggle as a rookie and turn it on later in their careers.