How old are Scout and Jem by the end of the book To Kill a Mockingbird?

During lengthy descriptions of her ancestry, her father's career, and the death of her mother, Scout reveals that Jem is four years older than she is. In fact, their mother died when Scout was two and Jem was six; but, the bulk of the storytelling starts when Jem is ten years old and Scout is six. They meet Dill for the first time in chapter one, which covers the summer before Scout enters first grade...

During lengthy descriptions of her ancestry, her father's career, and the death of her mother, Scout reveals that Jem is four years older than she is. In fact, their mother died when Scout was two and Jem was six; but, the bulk of the storytelling starts when Jem is ten years old and Scout is six. They meet Dill for the first time in chapter one, which covers the summer before Scout enters first grade in the fall. Jumping ahead to Chapter 26 we discover the following:

"School started, and so did our daily trips past the Radley Place. Jem was in the seventh grade and went to high school beyond the grammar-school building; I was now in the third grade, and our routines were so different I only walked to school with Jem in the mornings and saw him at mealtimes" (241).

The book comes to an end in chapter 31 around Halloween time. That places Jem in 7th grade around the ages of 12 or 13; and Scout is in the third grade around the ages of 8 or 9. Therefore, To Kill a Mockingbird covers about three years time from start to finish.