William Faulkner uses the mentally challenged Benjy to narrate the first part of The Sound and the Fury. I just finished reading Emma Donoghue's Room. In this novel, Donoghue uses the five-year-old Jack to tell a very adult story. Anna Seabold's The Lovely Bones is narrated by a murder victim. In other words, authors have selected very young narrators, mentally challenged narrators, even narrators who are no longer living. So, I suppose the answer to your question is that there is no inappropriate choice for a narrator. A better question to pose perhaps is why the author chose to tell the story from a particular point of view. What is gained by having a certain narrator tell the story, and how would the story change if another point of view were chosen?
An author has several choices in selecting a narrator. He or she can use an omniscient narrator and reveal the thoughts of several characters (Morrison's Beloved, for example, or Golding's Lord of the Flies). An author might use the main character of a story to tell the events (Twain's Huckleberry Finn). Or, an author might use a minor character to tell the story (Kesey's One Flew Over the Cuckoo's Nest). Each perspective works to develop theme, create suspense, or to depict characters.