To begin to act or seem more adult-like; to grow up or behave in a mature way.
彼は大学に入ってから急に大人じみてきた。
He suddenly started acting more grown-up after entering university.
その子の発言は以前より大人じみている。
That child's remarks are more adult-like than before.
時には大人じみた対応が必要だ。
Sometimes a mature response is necessary.