Act your age!

act (one's) age

To act 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 friends a bit.
See also: act, age

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!
See also: act