I can only be young once. But I can be immature all my life !

You can only be young once but you can always be immature.

He would like to get on with it, but he's young and he's immature and he doesn't understand how the process works, and so patience is going to be a virtue in this entire process for everyone.

You can only be young once. But you can always be immature.

You're only young once, but you can be immature forever.

He was like all freshmen: young, a little bit immature. We had to talk to him, stay on him and be sure he could follow the team rules.