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.