noun
adult guardian
Legal term for a person appointed to support and make decisions for an adult who lacks mental capacity.
彼は認知症の母親の成年後見人に選任された。
He was appointed as the adult guardian for his mother with dementia.
後見人 is a general guardian, often for a minor. 成年後見人 specifically refers to a guardian for an adult.
Compound of 成年 (adulthood) + 後見人 (guardian).