Why does the adult Scout begin her narrative with Jem's broken arm and a brief family history?

In the book "To Kill a Mockingbird" Scout has grown and matured and wants to bring the reader to the events of the year that had changed her life and had matured Jem and her.  She begins the story by telling about Jem's arm so that the reader will have an understanding of the children's ages, the kind of person Jem is, and her relationship with her brother.

The background of the town and the interaction of the people as well as how Scout first viewed her own is also central to the theme of change.  The reader believes that it is a nice quiet and comfortable sleepy little town.  However, as Scout matures through the events of Tom Robinson's trial, the reader and Scout begin to see the ugliness that lies beneath in the behavior and prejudice of the townspeople.