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. → overgrown
Examples from the Corpus
overgrown schoolboy/child• Eddie is an overgrown schoolboy with a penchant for pulling fleshy faces. 