Region: General | Usage example: He was told to act his age during the serious symposium.

Act your age

Meaning

Behave at a maturity level expected for your age.

Used when someone is acting too immature or irresponsible.

Origin notes can vary by region and source; this page gives the commonly reported explanation and usage context.

Etiquette guidance telling people to match behavior with age.

Appears in 20th-century parenting and schooling advice.