To grow up or become an adult; to move from childhood to adulthood physically or mentally, often implying increased responsibility or maturity.
子どもはいつか大人になる。今は好きにさせてあげたい。
Children will become adults someday. I want to let them be themselves for now.
大学に入ってから急に大人になったように感じる。
I feel like I suddenly became an adult after entering university.
責任感を持つことが大人になるということだ。
Having a sense of responsibility is what it means to become an adult.