1An adult — someone fully grown, both physically and mentally; in legal contexts, a person who has reached the age of majority.
日本では18歳から成人として扱われる。
In Japan, people are treated as adults from age 18.
このゲームは成人向けに作られている。
This game is made for adults.
成人の一日の摂取カロリーは個人差が大きい。
The daily calorie intake for adults varies greatly from person to person.
未成年ではなく成人であることを証明するため、身分証を見せた。
I showed my ID to prove I was an adult, not a minor.
2To become an adult; to grow up and reach adulthood. Can also refer to the associated event, as in 成人式 (coming-of-age ceremony).
二十歳になり、正式に成人した。
I turned twenty and officially became an adult.
成人したら、自分で判断して契約を結べるようになる。
Once you become an adult, you can make your own decisions and enter into contracts.
毎年一月に、成人したばかりの若者を祝う式典が開かれる。
Every January, a ceremony is held to celebrate young people who have just come of age.