act one's age, not one's shoe size

In reference to a person's shoe size (in the US or UK shoe size systems) usually being much lower than their age.

act one's age, not one's shoe size (third-person singular simple present acts one's age, not one's shoe size, present participle acting one's age, not one's shoe size, simple past and past participle acted one's age, not one's shoe size)

1. (idiomatic, usually imperative) Synonym of act one's age (“to behave in a mature manner; to avoid being childish”).

   You should act your age, not your shoe size!