The novel has a very serious and 'adult' theme, racial prejudice, but the events are presented through the eyes - and very often ears - of a child, or you could say three children, as their world is the prism through which we experience the novel. Therefore the events of the novel are a learning experience for the children and they have to confront some very serious realities in a relatively short period of time - adult hypocrisy, racial tension, violence, human cruelty, parental indifference in the case of Dill, death. Everything is an education for them and it is interesting that in the only two instances in the novel where school education is portrayed it is if the lesson is that school is not where you learn - except indirectly - but that life is what teaches you and helps you to grow up.
The growing up theme is very prominent because as Scout and Jem grow older, not only do their feelings change, but they also understand the views of society better. When they were children they believed that Boo was not real and he was a sort of game to them. However, when they got older they realized that Boo was a person and he was just secluded from society. They also gained a stronger sense of morality.