noun
one's age; age-appropriate behavior
Often used in the phrase 年甲斐もない (immature for one's age) or 年甲斐もなく (acting inappropriately for one's age).
年甲斐もなく、彼はゲームに夢中になっている。
Despite his age, he is absorbed in the game.
そんなことを言うなんて、年甲斐もない。
Saying such a thing is unbecoming of your age.
年齢 is a neutral term for age, while 年甲斐 carries a nuance of age-appropriate maturity or behavior.
Compound of 年 (toshi, 'year, age') and 甲斐 (kai, 'worth, effect'). The reading toshigai reflects rendaku voicing of kai.