大人おとなげない

i-adjective

immature; childish; not acting one's age

Describes behavior or attitude that is unbecoming of an adult; often used critically.

• 彼かれの大人おとなげない態たい度どに周まわりは呆あきれた。

  Everyone was disgusted by his childish attitude.

• そんなことで怒おこるなんて大人おとなげないよ。

  It's immature to get angry over something like that.