expression, i-adjective
unbecoming of one's age; not acting one's age; unworthy of one's years
Used to criticize behavior, appearance, or attitude that is considered inappropriate for someone's age, often implying immaturity or lack of dignity. Commonly used in the phrase 年甲斐もなく.
年甲斐もなく、彼はゲームに夢中になっている。
He is absorbed in video games, which is unbecoming of his age.
そんな派手な服を着るなんて、年甲斐もないと思わない？
Don't you think wearing such flashy clothes is not acting your age?
大人げない means 'childish' or 'immature' and focuses on lack of adult composure, while 年甲斐もない specifically emphasizes that the behavior is inappropriate for one's age, often with a nuance of disgrace.
年相応 means 'age-appropriate' and is often used positively or neutrally, whereas 年甲斐もない is always negative, criticizing something as not befitting one's age.
From 年 (toshi, 'age') + 甲斐 (kai, 'worth, effect') + もない (mo nai, 'not even'). Literally 'not even worth one's age', meaning that one's actions do not reflect the maturity expected at that age.