to look (sound, behave) grown-up; to be adult-like; to mature
彼の声は急に大人びた。
His voice suddenly sounded grown-up.
妹は高校に入ってから大人びている。
My younger sister has been looking more mature since starting high school.
弟や妹がいる子は早く大人びる傾向がある。
Children with younger siblings tend to behave more grown-up early on.