[act] {one's} age

to behave in way that is expected of someone of a particular age

He needs to act his age and take on more adult responsibilities.
It's time for her to act her age and stop behaving like a teenager.
When attending formal events, it's important to act your age and show professionalism.
Despite being in their fifties, they still know how to have fun and act their age when appropriate.
She was praised for acting her age and handling the situation with maturity.