The reader is given an exact year for the setting of To Kill a Mockingbird during the trial of Tom Robinson. While delivering his closing arguments, Atticus reveals that it is 1935. Bob Ewell's attack on the Finch children, resulting in Jem's injury, takes place on Halloween of that same year. As the voice of the narrator, an adult Jean Louise tells us that Jem was nearly 13 when his arm was broken; however, she never reveals whether he was born at the end of one year or the beginning of the next. Therefore, doing a bit of arithmetic will lead the reader to conclude that Jem was born sometime at the end of 1922 or the beginning of 1923. Because Jean Louise is four years younger, her birth year would have been 1926 or 1927. The book covers a three-year span, so the beginning takes place in 1932. Harper Lee did a wonderful job of making her story come full-circle!
The novel takes place during the 1930s although a specific year is not given. Jem's birthdate is not given either, but the reader can assume that the year he was born in was in the 1920s because chapter one states that Scout was almost six and Jem was nearly ten.
The novel spans a period of three years, so Jem is thirteen at the end of the novel. The first line of the book tells the reader that Jem "was nearly thirteen [when he] got his arm badly broken at the elbow" (Lee). This is referring to the end of the story when Jem and Scout are attacked.