When he was almost thirteen, my brother Jem got his arm broken badly at the elbow.
The beginning of the novel opens with this statement from Scout, in which she provides the reader characterization of her brother Jem. From the beginning, the reader can infer how much Scout looks up to her older brother, who is four years older than her.
"I maintain that the Ewells started it all, but Jem, who was four years my senior, said it started long before that."
By doing a little simple math, the reader can conclude that when Jem broke his arm at the age of thirteen that Scout would have been nine years old. As the true action of the story begins, Jem is actually ten, and Scout is six, the consummate tag-along little sister.