别孩子气！成熟点吧！
said to someone to tell them to stop behaving like someone who is much younger
Boys, act your age, please!
You need to grow up and act your age!
You're too old for silly pranks like this. I really wish you would just act your age.
Come on, act your age! That food is for eating not for playing with!
Just act your age and apologize to her!
See also: age noun (TIME SPENT ALIVE)