The narrator is an adult Scout looking back on the events of her childhood that made her the person she is.
The narrator at the beginning of the book is a woman named Jean Louise Finch. As a child, she was called Scout.
The first paragraph of the novel is interesting, because it is clearly from the perspective of an adult. We learn a lot from this one paragraph. We learn that the narrator is older, looking back on an incident of long ago. We learn that something happened to injure Jem, and it was something significant. We also learn that Jem did not really care as long as he could play ball.
As the narrative continues, we learn more about this important incident.
When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to his accident. I maintain that the Ewells started it all, but Jem … said it began the summer Dill came to us, when Dill first gave us the idea of making Boo Radley come out. (ch 1)
We do not know how long “enough years” are for them to look back, but we can assume Scout and Jem are at least adults. They are equals at this point, and four years makes no difference now. It only mattered when they were children.
The narrative soon switches to a much younger six year old Scout and ten year old Jem. This is important because we need to see things from a child’s perspective. It is one of the morals of the book that adults are jaded, and only children can see the world for what it really is.