A first example given is based on the fact that the narrator directly tells us she is an adult and this is about when she was a child. On the very first page, our narrator says:
When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to his accident.
This quote demonstrates that years had past between the incidents that had happened and the current time from which the author is writing. This leads us to believe that as the text continues, the speaker is now an adult, but writes of a time as a child.
As the text continues in the first chapter, we can see that the language used to narrate includes some historical perspective, psychological analysis of human behavior and adult vocabulary. Any given sentence can prove this just like this one:
Being Southerners, it was a source of shame to some members of the family that we had no recorded ancestors on either side of the Battle of Hastings. All we had was Simon Finch, a fur-trapping apothecary from Cornwall whose piety was exceeded only by his stinginess.