In To Kill a Mockingbird, who broke his arm at the elbow the summer before he turned thirteen?

Scout begins the story by discussing the events that led up to her brother, Jem, breaking his arm at the elbow when he was nearly thirteen years old. Throughout the entire novel, Scout retells the story in retrospect of how her father, Atticus, valiantly defended Tom Robinson against a prejudiced jury. Bob Ewell accused Tom of raping his daughter Mayella. Although Atticus and Tom lost the case, Bob's reputation was permanently ruined when it was revealed he actually assaulted his daughter. Bob vows to get revenge against Atticus following the trial, and in Chapter 28 he attempts to murder Atticus's children. When Jem and Scout are walking home from Maycomb's Halloween festival, Bob Ewell viciously attacks them. During the struggle, Bob Ewell throws Jem to the ground and begins to wrestle with him. Scout says,

"We were nearly to the road when I felt Jem’s hand leave me, felt him jerk backwards to the ground. More scuffling, and there came a dull crunching sound and Jem screamed" (Lee 161).

While Bob is wrestling with Jem, he breaks Jem's arm. Fortunately, Boo Radley intervenes and saves the children. Jem fully recovers from his injury, but Scout mentions that his left arm hangs slightly shorter than his right. She also says Jem isn't self-conscious about his injury because he can still pass and punt.