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
You're old enough to know better—don't cry over something like that.
Despite being old enough, he still depends on his parents.
Alternative spelling using 歳, which is also common for 'age' and carries a slightly more formal or literary feel.
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.