What happened to Jem when he was nearly thirteen in To Kill a Mockingbird

The opening sentence of the book reads, "When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow." (Lee 3) Harper Lee begins the books by introducing the narrator Scout, who says that she and Jem sometimes look back to discuss the events that led up to how he broke his elbow. The remainder of the novel depicts the various events and moments in the lives of the Finch children that lead to when Jem breaks his elbow. In Chapter 28, Bob Ewell attacks Jem and Scout while they are walking home from Maycomb's Halloween festival. Bob is attempting to enact revenge on Atticus for ruining his reputation in the Tom Robinson trial. Arthur "Boo" Radley, the Finches reclusive neighbor, saves the children by defending them during the attack. Boo Radley is able to use Bob Ewell's knife against him and stabs Bob during the struggle. After the fight is over and Bob Ewell is dead, Boo Radley carries Jem to his house. Jem is unconscious and broke his arm near the elbow during the attack.