noun
adult; person of full age
Formal or legal term for a person who has reached the age of majority. In everyday speech, 大人 (おとな) is more common.
日本では、成年者は20歳以上と定義されていた。
In Japan, an adult was defined as a person aged 20 or older.
成年者としての権利と責任を理解する必要がある。
It is necessary to understand the rights and responsibilities as an adult.
未成年者 means 'minor' or 'person under legal age', the direct antonym of 成年者.
Compound of 成年 (せいねん, 'full age, adulthood') + 者 (しゃ, 'person'). The term is a straightforward Sino-Japanese construction.