mature age; advanced age
Refers to being of an age where one is expected to act responsibly; often used in the phrase いい年をして to criticize childish behavior.
Don't cry over something like that at your age.
age when one is old enough to know better
Specifically the age at which one should be mature; almost always used in the set phrase いい年をして to scold or express disbelief at someone's immaturity.
See also: いい年をして
He's already old enough to know better, but he still depends on his parents.
You shouldn't tell lies like that at your age.
Standard kanji spelling; 年 is the most common character for 'age' in this phrase.
Alternative spelling using 歳, which specifically means 'age' and is also common.
Rare spelling using the kanji for 'good'; not commonly used in modern writing.
Compound of いい (good) and 年 (age). The phrase literally means 'good age' but is used idiomatically to refer to an age at which one should be mature.