How does Jem come of age in To Kill a Mockingbird?

In To Kill a Mockingbird's opening, Harper Lee provides a hint for what leads to the loss of Jem's innocence. Scout narrates,

"When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow. . . . When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to his accident" (Lee 3).

Lee's choice to begin her seminal work in this manner demonstrates that whatever happened to Jem when he was thirteen is pivotal in his development into a young man. Of course, near the book's conclusion, readers discover that Jem's arm is broken in his tussle with Bob Ewell, but that event follows the real impetus for Jem's maturation.

As Jem, Scout, and Dill watch the trial of Tom Robinson play out before them, Jem becomes steadfastly convinced that Atticus has won the case. He tells Reverend Sykes that there is no way that Tom will be convicted, and despite the reverend's warning that trials do not always end justly, Jem's confidence in his father, in the truth, and in the judicial system lead him to believe that when the jury comes back from deliberation, it will be with an acquittal for Tom Robinson. However, as Judge Taylor polls the jury, and each one votes "guilty," Jem white-knuckles the balcony rail and takes each "condemnation" of Tom as a stab to his own back. While Scout is left bewildered, not quite understanding the gravity of what has just happened, Jem breaks down in tears and storms home. He tells Atticus that "it ain't right" and then asks his father, "How could they do it, how could they?" (Lee 213).

When Jem hears that guilty verdict, he loses his sense of moral justice, his faith in his neighbors, and his confidence in truth's victory. He is a much more somber Jem as the novel continues which eventually enables him to stand up to Bob Ewell when his and Scout's lives are in danger.