大人おとなぶる

godan verb (-ru)

act like an adult; pretend to be grown-up

Often used for children or young people trying to appear mature, sometimes with a slightly negative or mocking tone.

• 弟おとうとは大人おとなぶって難むずかしい本ほんを読よんでいる。

  My younger brother is reading a difficult book, acting like an adult.

• 彼かの女じょはまだ高こう校こう生せいなのに、大人おとなぶってワインを飲のもうとした。

  Even though she's still a high school student, she tried to drink wine, pretending to be an adult.