be one's ageSee: ACT ONE'S AGE.
{v. phr.} To do the things that people expect someone of your age to do, not act as if you were much younger than you are.
Mr. O'Brien was playing tag with the children at the party. Then Mrs. O'Brien said, "Henry! Act your age!" and he stopped. [TahlilGaran] English Idioms Dictionary ▲