In To Kill a Mockingbird, Harper Lee uses the voice of an adult Jean Louise to reminisce on her own childhood as the tomboy "Scout." Lee makes us aware of the narrator's maturity within the first two paragraphs of the novel. The opening line of the book is the first clue: "When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow." Because this line is written in the past tense, we know that Jem is now beyond thirteen years old.
But what does this tell us of the narrator's age? Well, she begins the second paragraph by stating that many years have passed since that accident. This information, added to the fact that Jem's accident occurred at the age of thirteen, implies that both Jem and Jean Louise are adults at the time the story is being told.
Another clue, aside from the description of the years that have passed, is the level of language the narrator uses. Though Jean Louise is describing the events through the eyes of her childhood, she uses the language of an adult.