expression, noun
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.
いい年をして、そんなことで泣くなよ。
Don't cry over something like that at your age.
彼はもういい年なのに、まだ親に頼っている。
He's already old enough to know better, but he still depends on his parents.
expression, noun
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
いい年をして、そんな嘘をつくものじゃない。
You shouldn't tell lies like that at your age.
年甲斐 also refers to age-appropriate behavior, but is used in phrases like 年甲斐もなく (unbecoming of one's age) and emphasizes the dignity expected of one's years.
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.