Why does Ivan tell his daughter to "act her age"?

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.