The notion of growing up over time is of vital importance in Lee's work. Scout is one of the most dominant examples of this. The novel's structure is one of growing up as Scout does the narration. In doing so, it becomes clear that Scout's psychological emergence is essential to the novel's meaning. The novel's opening lines speak to this condition:
Maycomb was an old town, but it was a tired old town when I first knew it. In rainy weather the streets turned to red slop; grass grew on the sidewalks, the courthouse sagged in the square. Somehow it was hotter then: a black dog suffered on a summers day; bony mules hitched to Hoover carts flicked flies in the sweltering shade of the live oaks on the square. Men's stiff collars wilted by nine in the morning. Ladies bathed before noon, after their three-o'clock naps, and by nightfall were like soft teacakes with frostings of sweat and sweet talcum.
The novel's opening lines indicate how Scout has grown up. The rest of the novel is devoted to exploring this process of maturation and emotional growth. Scout's recollection of Maycomb help to establish that what is being read is a memoir of sorts about her growing up and the maturation which becomes a part of her identity.
As the novel progresses, specific instances remind the reader that Scout is growing up. While she might initially object to what Atticus says, part of her process of growing up involves recognizing the truth of Atticus's words. At specific moments, it becomes clear that Scout grows up through the realization of what her father says is correct and sound moral and ethical advice:
Jem stayed moody and silent for a week. As Atticus had once advised me to do, I tried to climb into Jem's skin and walk around in it: if I had gone alone to the Radley Place at two in the morning, my funeral would have been held the next afternoon. So I left Jem alone and tried not to bother him.
Harper Lee shows through Scout that part of what it means to grow up is to recognize when the adults who show care are proven to be right. Scout also grows up in terms of how others see her, as seen in how Uncle Jack recognizes her wisdom:
I know. Your daughter gave me my first lessons this afternoon. She said I didn't understand children much and told me why. She was quite right. Atticus, she told me how I should have treated her-oh dear, I'm so sorry I romped on her.
Scout has grown over the course of the novel in how she comes to view herself, in how she views others, and finally in how others view her. Through absorbing Atticus's teachings regarding climbing into the perceptions of another person and "walk around in it," Scout grows up. In this transformation and growth of character, Scout grows up during the course of time in To Kill a Mockingbird.
Growing up is also a part of Jem's identity in the course of the novel. As Scout does, Jem learns to understand more of himself, the world, and his place in it as part of the process of growing up. One part of this process is establishing himself socially apart from Scout:
His maddening superiority was unbearable these days. He didn't want to do anything but read and go off by himself. Still, everything he read he passed along to me, but with this difference: formerly, because he thought I'd like it; now, for my edification and instruction.
Jem shows the process of growing up in terms of changing over time, something that Scout further confirms when she says both of them "began to part company" and describing him as "allergic" to her presence in public. His social distance from Scout is an example of him growing up. Like Scout, Jem also grows up in terms of understanding the truth regarding Atticus's lesson. At the same time, Jem also grows up in how he recognizes mistakes in judgment. As he gets older, his desire to recognize the importance of image and how he is seen becomes a part of his identity. Jem grows up in how he does not want to be seen in a negative light by Atticus: "I—it's like this, Scout," he muttered. "Atticus ain't ever whipped me since I can remember. I wanta keep it that way. […] We shouldn'a done that tonight, Scout." Jem grows up in the way he recognizes the importance of his image and how others, namely Atticus, sees him. In being conscious of his perception in the eyes of other people, Scout shows an aspect of growing up and how the process of maturation takes place over the course of time in the novel.