In the short story “The Moustache” by Robert Cormier, the lesson is to cherish childhood.
Mike, the main character of the story, has a moustache. His family wants time to shave it off, but he wants to keep it. His grandmother has dementia, and he goes to visit her because he feels guilty about not doing so. When he comes there, her grandmother thinks that he is his grandfather, who is also named Mike. She says that she wants to apologize for accusing him. When Mike gets home, he shaves off his moustache.
Mike got a moustache to prove that he could, and he kept the moustache because he thinks it makes him look like an adult. However, when he realizes what being an adult is like, he shaves it off.