act (one's) age

Translations
вести себя соответственно возрасту
He should act his age and stop playing childish pranks.
Ему следует вести себя соответственно возрасту и перестать устраивать детские шалости.
Definitions
To behave in a manner that is appropriate for one's age, often implying that someone is behaving immaturely or irresponsibly for their age.