expression
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.
いい年してそんなことで泣くなよ。
Don't cry over something like that at your age.
いい年して、まだ親に頼っている。
Even though he's old enough to know better, he still depends on his parents.
Similar meaning but more formal and less colloquial; often used in written language.
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.