overgrown schoolboy/child
From Longman Dictionary of Contemporary Englishovergrown schoolboy/childovergrown schoolboy/childBAD BEHAVIOUR OR ACTIONSan adult who behaves like a child – used to show disapproval Stop acting like an overgrown schoolboy. → overgrownExamples from the Corpusovergrown schoolboy/child• Eddie is an overgrown schoolboy with a penchant for pulling fleshy faces.