act/be 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.
act your age
If you tell someone to act their age, you are telling them to behave in a way that is suitable for their age, because they are behaving in a childish way. One patient complained that the nurse had told her: `Act your age and pull yourself together.' Note: This expression is used to express disapproval of someone's behaviour.