大人おとなっぽい

i-adjective

grown-up; mature; adult-like

Describes a person, behavior, appearance, or atmosphere that seems more mature or adult than expected, often with a nuance of precociousness or sophistication. Can be used positively or slightly critically.

• 彼かの女じょはまだ高こう校こう生せいなのに、話はなし方かたが大人おとなっぽい。

  She's still a high school student, but her way of speaking is mature.