At around the 10 minute mark in Author! Author! there's a dinning scene. When one of Ivan's daughter said "My cheese, I love my cheesie" And Ivan told her to act her age (she's 11). What does that mean?
She's saying something that's very childish, that someone much younger (say five or six years old) might say.
She's being told to act her age, i.e. act like a twelve years old, not someone of a much younger age.
Note: You've asked a few questions of this type recently. I'd strongly suggest checking out the Stack site for English Language Learners, where these types of questions are probably better suited.