When the book opens, the narrator tells us about an accident that happened to her brother when he was thirteen. He broke his arm badly at the elbow. She proceeds to tell us that it healed, but after that his left arm was a little shorter than his right, however, he still was able to play football. Therefore the reader knows that some time has passed since that event. She is able to tell us what happened after the serious break of the arm.
In the second paragraph, she writes,
"When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to this accident." (pg 3)
Now we know that enough years have passed for the children to have contemplated the incident. Each person, Jem and Scout, has a different theory of how it started. Scout thinks it started with the Ewells, but Jem, who is four years older than she is, thinks it started the summer Dill first came to Maycomb. However, they were older now and Scout writes,
"We were far too old to settle an argument with a fist-fight so we consulted Atticus. Our father said we were both right." (pg 3)
So we know that they are too old to fist-fight, and I am sure too refined. She then continues her story starting with the history of her family.