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.
Children eventually become adults.
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.
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.
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.
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.
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.
It's about time you grew up (acted like an adult).
Explicitly refers to becoming mentally or emotionally mature.
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.
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.
He hasn't become an adult yet. (casual)
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).
We celebrated Coming of Age Day with family.