It's bad to be immature.
Tim goes to the park and sees a girl he likes.
Tim wants to interact with the girl.
Tim interacts with the girl in a normal and mature manner, making small talk.
Tim acts in a silly and childish way with the girl, making jokes suited more to small children.
The girl thinks Tim is a normal guy and quite mature for his age.
The girl thinks Tim is very childish and that he should act his age.