The quote to which you are referring:
You know something, Scout? I've got it all figured out, now. I've thought about it a lot lately and I've got it figured out. There's four kinds of folks in the world. There's the ordinary kind like us and the neighbors, there's the kind like the Cunninghams out in the woods, the kind like the Ewells down at the dump, and the Negroes.
This shows his immaturity, naturally, because he thinks he is superior. As a young adolescent, he is just trying to figure things out for himself, and he has become aware of prejudice all around him. He also realizes that the Ewells are part of the lower hierarchy of society.
If you read Scout's response, you will see that she actually has a more mature grasp of life.
Someone answered a similar question right here on eNotes. See the link below.