One way that Harper Lee illustrates Jem's and Scout's coming of age is through their developing perception of Boo Radley. At the beginning of the story, Scout views Boo as a "malevolent phantom," and Jem believes that he is a menacing creature who eats raw animals and is covered in blood. As the novel progresses, Jem discovers that Boo mended and folded his pants and that Boo has been giving them small gifts in the knothole of the Radley tree. After Jem discovers that the knothole has been filled in with cement, he cries on the front porch at the lost opportunity to create a friendship with Boo.
Jem's ability to perceive Boo as a compassionate, kind neighbor illustrates his maturation. Toward the end of the story, Scout also demonstrates her coming of age by viewing Boo in a new light. She tells Jem:
Boo doesn’t mean anybody any harm, but I’m right glad you’re along (Lee 135)
Scout sympathizes with her neighbor and understands that he is a harmless, reclusive man. After Boo saves her life from Bob Ewell's vicious attack, Scout walks Boo home and says:
Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies, and our lives (Lee 148)
Harper Lee also depicts the coming-of-age theme through Scout's understanding of justice and her ability to comprehend and apply Atticus's life lessons. In chapter 10, Atticus tells the children that it is a sin to kill a mockingbird. Miss Maudie elaborates on Atticus’s comment by saying,
Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird (Lee 49)
Later in the story, Scout and Jem lose their childhood innocence after witnessing racial injustice firsthand during the Tom Robinson trial. Following the trial, Scout begins to display her maturation and moral development by recognizing hypocrisy and viewing innocent, vulnerable individuals as symbolic mockingbirds. She even understands the concept of Mr. Underwood’s editorial regarding Tom’s fate and realizes that the jury’s prejudice was solely responsible for Tom’s wrongful conviction.
After Boo Radley saves her life during Bob Ewell’s attack, she overhears Sheriff Tate explaining why he will not inform the community of Boo's heroics—in order to protect him from the public limelight. When Atticus asks Scout if she understands Tate's reasoning, Scout displays her maturation by metaphorically applying Atticus's earlier lesson regarding mockingbirds. Scout asks her father:
Well, it’d be sort of like shootin‘ a mockingbird, wouldn’t it? (Lee 147)
Scout displays her coming of age and moral development by no longer fearing Boo Radley, by exercising perspective, by understanding the concept of justice, and by applying her father's life lessons.