In her novel To Kill a Mockingbird, why does Harper Lee present the theme of growing up?

One conclusion that can be drawn is Harper Lee saw a parallel between a child coming of age and the South growing and changing. Therefore, Lee decided to present the need for the South to grow and develop by capturing Jem's and Scout's development as they matured as children. In paralleling the children's growth with the South's growth, Lee presents the theme of the need to mature in To Kill a Mockingbird.

Just as the children undergo many changes as the book progresses, the town of Maycomb, representative of all small Southern towns, also undergoes many changes even though these changes are just baby changes. At the opening of the book, Scout describes Maycomb as a "tired old town." It is tired for several reasons: (1) it is in the midst of the Great Depression; therefore, there is no money to produce a great deal of activity within the town; (2) being rural, the town looks a bit worn out and run-down with its grass growing on the sidewalks and its courthouse sagging; and (3) its people are very stuck in their ways. Scout characterizes Maycomb's people as being stuck in their ways when she describes the men as, despite the heat, persisting in wearing stiff collars that were "wilted by nine in the morning" and ladies persisting in bathing and napping traditions.

Author Lee also uses several characters to describe Maycomb as being stuck in its ways such as Aunt Alexandra, who persists in holding the racist and sexist views of a Southern Belle, and
Mrs. Henry Lafayette Dubose, who is as equally racist as Aunt Alexandra and holds equally strict views about how children should behave. Interestingly, many of Aunt Alexandra's notions are rejected by Atticus, such as her view that Calpurnia should be dismissed because Finch children should not be being raised by an African-American woman, and Mrs. Dubose passes away. Aunt Alexandra's rejected notions and Mrs. Dubose's passing both symbolize the ensuing death of old Southern ways and beliefs. Hence, as the story progresses, the tired old town that is stuck in its ways begins to be challenged and to change a tiny bit.

The town's change is further symbolized by the children's success in disbursing the lynch mob and in the fact that Robinson's jury was out for so long before returning with the guilty verdict. Miss Maudie reflects on the subtle changes of the town after the trial when she says the following to the children:

[A]s I waited I thought, Atticus Finch won't win, he can't win, but he's the only man in these parts who can keep a jury out so long in a case like that. And I thought to myself, well, we're making a step--it's just a baby-step, but it's a step. (Ch. 22)

Hence, just as Scout and Jem mature a great deal as a result of what they experience due to Robinson's arrest and trial, Maycomb also matures just a tiny bit. Therefore, it can be said that Lee uses the parallel of the children's growth with the town's growth to point to the South's need to continue to grow and develop.