expression
despite one's age; old enough to know better
Used to criticize or express surprise at someone's immature or inappropriate behavior, implying they should act more their age. Often followed by a negative comment.
See also: いい年して
いい年をして、そんなことで泣くなよ。
You're old enough to know better—don't cry over something like that.
いい年をして、まだ親に頼っている。
Despite being old enough, he still depends on his parents.
A slightly more casual or clipped version of いい年をして, with the same meaning and usage.
From いい (good, here meaning 'considerable' or 'old enough') + 年 (age) + をして (even though doing at that age). The phrase literally means 'even though being of a good age', used sarcastically.