Translation guide
In Japanese, expressing 'become an adult' depends on whether you mean reaching legal adulthood, maturing emotionally, or simply growing up. The most common and natural way is to use 大人になる (otona ni naru). Other expressions exist for formal, legal, or coming-of-age contexts.
To become an adult in the general sense of growing up, reaching adulthood, or acting like an adult.
The most common and natural way to say 'become an adult'. It covers physical, legal, and emotional adulthood depending on context.
子供はいつか大人になる。
Children eventually become adults.
大人になったら、何になりたい？
What do you want to be when you grow up?
Specifically means to reach the legal age of adulthood (currently 18 in Japan). More formal than 大人になる.
日本では18歳で成人する。
In Japan, you become an adult at 18.
A slightly poetic or celebratory phrase meaning 'to join the ranks of adults'. Often used in coming-of-age contexts.
二十歳になって、大人の仲間入りをした。
Turning 20, I joined the ranks of adults.
To participate in the formal coming-of-age ceremony (Seijin Shiki) or to reach the age of majority in a ceremonial sense.
Specifically refers to celebrating or participating in the Coming of Age ceremony. This is the most natural way to talk about the event.
今年、成人式を迎える。
I will celebrate my coming of age this year.
To celebrate Coming of Age Day (a national holiday). Less about the personal ceremony and more about the day itself.
成人の日を家族で祝った。
We celebrated Coming of Age Day with family.
To become mature in behavior or attitude, to act like an adult rather than a child.
Also used for emotional or behavioral maturity. Context makes it clear.
そろそろ大人になりなさい。
It's about time you grew up (acted like an adult).
Explicitly refers to becoming mentally or emotionally mature.
彼は精神的に大人になった。
He has matured emotionally.
Means 'to grow up' or 'to mature' in a broader sense, including physical, mental, or personal growth. Not limited to adulthood.
人は経験を通して成長する。
People mature through experience.
大人になる is the everyday, all-purpose expression for becoming an adult. 成人する is formal and specifically refers to reaching the legal age of majority. Use 成人する in official or legal contexts, and 大人になる in casual conversation.
彼はまだ大人になっていない。
He hasn't become an adult yet. (casual)
彼はまだ成人していない。
He has not yet reached the age of majority. (formal)
There is no verb 大人する. Always use 大人になる (otona ni naru) or 成人する (seijin suru).