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adult; grown-up
becoming an adult; coming of age; growing up (to be a man, woman)
彼女は来年で二十歳の成人を迎える。
She will become an adult next year.
成人になったら責任が増える。
Responsibilities increase when you become an adult.
成人式は日本の伝統的な行事だ。
The coming-of-age ceremony is a traditional Japanese event.