Final answer:
Jem was 13 years old when he broke his arm in the novel 'To Kill a Mockingbird' by Harper Lee.
Step-by-step explanation:
In the excerpt from To Kill a Mockingbird, we learn that Jem was 13 years old when he broke his arm. This detail is part of the novel's opening where Scout Finch narrates the events retrospectively, offering insights into the life and experiences of her and her brother, Jem Finch. The novel, written by Harper Lee, is a classic piece of American literature that addresses complex themes such as racism, innocence, and moral growth.