Our first-person narrator is Scout Finch, who is five when the story begins and eight when it ends. From the first chapter, though, it’s clear that Scout is remembering and narrating these events much later – after all, the second paragraph of the novel begins, "When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to [Jem’s] accident" (1.2).
For the most part, Scout recounts the events from her childhood perspective, as she understood them at the time, rather than imposing an adult commentary. This makes the narrative perspective a naïve one: often we get descriptions of events just as she experiences them, without commentary on what they mean, or a commentary that is humorously innocent. But having the adult perspective be there in the background, even if it isn’t in play for most of the narration, means it can pop out when it’s needed to point out important things that the narrator realizes only later, to make sure that the reader sees them too.