To do the things that people expect someone of your age to do, not act as if you were much younger than you are.
Совершать поступки, которые люди ожидают; вести себя по взрослому.
Mr. O'Brien was playing tag with the children at the party. Then Mrs. O'Brien said, "Henry! Act your age!" and he stopped.
My niece is in her early thirties, but she prefers not to act her age.
Моей племяннице уже за тридцать, но она не желает вести себя по-взрослому.