The novel To Kill a Mockingbird is told in flashback by an adult Scout in the late 1950s looking back some twenty-odd years on when she was six in 1933. So, even though the narrator is an adult, she maintains a child-focused perspective in order to achieve a bildungsroman, a novel of education.
Indeed, Lee's coming-of-age novel focuses first on her childhood innocence and fascination with Boo Radley (in Part I) and then on her harrowing experiences during the Tom Robinson trial (in Part II). The novel is framed by the memory of her brother Jem's broken arm as its trigger (notice: "mockingbirds" Tom and Jem both have broken limbs (wings?), symbols of suffering and cruelty).
Overall, the novel is a children's / young adult book with some adult themes. It contains elements of the Southern Gothic and the Southern novel of manners. Though it is told in first-person, Scout focuses as much on Jem's coming-of-age as her own. The narrator certainly has a legal mind (Harper Lee and her father were both lawyers), not to mention that the novel lauds Atticus as a noble Populist and champion in grass-roots civil rights.
So says Enotes:
Scout Finch, who narrates in the first person ("I"), is nearly six years old when the novel opens. The story, however, is recalled by the adult Scout; this allows her first-person narrative to contain adult language and adult insights yet still maintain the innocent outlook of a child. The adult perspective also adds a measure of hindsight to the tale, allowing for a deeper examination of events. The narrative proceeds in a straightforward and linear fashion, only jumping in time when relating past events as background to some present occurrence.