Our first-person narrator is Scout Finch, who is five when the story begins and eight when it ends. From the first chapter, 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 gives us the events from her childhood perspective, as she understood them at the time, rather than imposing an adult commentary. This makes the narrative perspective naïve: often we get descriptions of events just as she experiences them, without commentary on what they mean, or a commentary that is hilariously 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. Like this:
Mr. Underwood didn't talk about miscarriages of justice, he was writing so children could understand. Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. (25.27)
Seven- or eight-year-old Scout doesn't understand words like "miscarriages of justice"; this is the adult Scout telling us that "miscarriage of justice" is what happened. She's giving us her kid's perspective: what happened was a sin. And that makes this book perfect for tweens and teens: old enough to understand the shades of justice, but still young enough to remember the black-and-white rules of childhood.