act one's age

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act one's age (third-person singular simple present acts one's age, present participle acting one's age, simple past and past participle acted one's age)

1. (idiomatic) To behave in a manner appropriate to one's age.

   Synonym: behave one's age

2. (imperative, originally US) To behave in a mature manner; to avoid being childish.

   Act your age and stop fiddling with that pen – you’re showing us up.

• act one's age, not one's shoe size