despite being old enough to know better; acting one's age
Used to criticize or express surprise at someone's immature or inappropriate behavior given their age. Often followed by a negative evaluation.
Don't cry over something like that at your age.
Even though he's old enough to know better, he still depends on his parents.
Standard kanji spelling using 年.
Variant using 歳, which emphasizes age in years.
Irregular kanji usage; 齢 is not standard for this expression.
From いい (good, considerable) + 年 (age) + して (doing), literally 'doing (something) at a considerable age'. The expression is idiomatic and its exact origin is uncertain.