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.
In Japan, an adult was defined as a person aged 20 or older.
It is necessary to understand the rights and responsibilities as an adult.
成年者 is the precise legal counterpart to 未成年者, denoting a person who has reached the age of majority.
minor; underage person
Compound of 成年 (せいねん, 'full age, adulthood') + 者 (しゃ, 'person'). The term is a straightforward Sino-Japanese construction.