Adulthood or the age of majority; the legal age at which a person is considered an adult.
日本では成年の年齢が18歳に引き下げられた。
In Japan, the age of majority was lowered to 18.
成年になると契約を自分で結べるようになる。
When you become an adult, you can enter into contracts on your own.
彼は来年で成年になる予定だ。
He is scheduled to reach adulthood next year.