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

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

Well, I still don't like him! What kind of a man doesn't name his boat?