How does Scout changing as a result of Jem's puberty (his changing persona) affect the novel To Kill a Mockingbird?
As Jem matures he desires less and less to be involved in all that Scout does. At times he adopts a tone of superiority towards her. "In addition to Jem's newly developed characteristics, he had acquired a maddening air of wisdom." (Ch.12) Frequently, Jem tells his little sister to leave him alone. Sometimes she turns to Calpurnia and talks with her. Calpurnia consoles Scout:
"Baby. . . I just can't help it if Mister Jem's growin' up. He's gonna want to be off to himself a lot now, doin' whatever boys do, so you just come right on in the kitchen when you feel lonesome" (Ch.12)
Further, Scout is offended by what she perceives as Jem's attitude of superiority to her. She feels somewhat demeaned by Jem's opinion that she "can't hold something in [her] mind but a little while" (Ch.14) while "grown folks" like Jem can think more deeply and analyze things.
At one point, Jem threatens to spank Scout. Scout hits him, and they fight seriously with one another until Atticus breaks them apart, sending both children to bed. But Scout feels something under her bed. Fearing that it is a snake, she forgets her momentary repulsion for her brother and asks Jem to come and see what is there. Jem exclaims when he sees that it is Dill. After a dirty Dill emerges, Jem observes that Dill's parents must not know where he is. Dill grins as he observes that they are "probably still searchin' all the picture shows in Meridian." When he hears this, Jem does something that appalls Scout. He tells Dill that he should let his mother know that he is at their house.
Then he rose and broke the remaining code of our childhood. He went out of the room and down the hall. "Atticus, . . . can you come here a minutes, sir?" (Ch.14)
Atticus enters the room and addresses Dill. He calmly tells Dill that he will phone Miss Rachel and inform her that Dill is at their house and can stay the night. Then, he drolly tells Dill to "put some of the county back where it belongs, the soil erosion's bad enough as it is." Atticus leaves the room, and Jem explains to Dill that he had to tell his father that he was in their house. Scout and Dill say nothing, but they "left him without a word." (Ch. 14)
Later on, Scout realizes that Jem is going through changes as he tries to make sense of the adult world. After the trial of Tom Robinson, Jem does not understand how the jury could convict Tom when evidence points to his innocence. Because he wants to protect the harmless, Jem even scolds Scout one day when she almost squashes a roly-poly bug.
In the beginning of the novel, both Scout and Jem are young and innocent and act like children. Yet Scout’s world is tied so closely with Jem’s that when he grows up, she grows up. She does not always understand his new maturity. Sometimes Scout describes Jem’s changes as confusing and even alarming.
Jem was twelve. He was difficult to live with, inconsistent, moody. His appetite was appalling, and he told me so many times to stop pestering him I consulted Atticus: "Reckon he's got a tapeworm?" Atticus said no, Jem was growing. (ch 12)
Jem is beginning to want to go his own way, and is less tolerant of childhood play. This marks a turning point in the book. Jem’s maturity really comes just in time, because the trial is starting and Scout and Jem both have to face the consequences of their father’s socially unacceptable choice to defend a black man.
Scout is a keen observer of human nature, and she keeps a close eye on her brother. Naturally she will start to notice his reaction to people who condemn Atticus and other adult topics like the trial. Scout also needs to grow up fast.