something that you say to someone who is being silly to tell them to behave in a more serious way.
Oh, act your age, Chris! You can't expect to have your own way all the time.
act (one's) age
To behave in accordance with one's expected level of maturity. Really, John, act your age a little. You're like a child sometimes!I'm glad that Sarah is so committed to her studies, but sometimes I wish she would act her age and go out with her friends a bit.
Act your age!
Behave more maturely! (A rebuke for someone who is acting childish. Often said to a child who is acting like an even younger child.) Child: Aw, come on! Let me see your book! Mary: Be quiet and act your age. Don't be such a baby!