You can apply the quote to several characters in To Kill a Mockingbird, and I would suggest mentioning those characters in your thesis statement and then using your body paragraphs to apply the quote to each of your chosen characters. See examples below.
1. Atticus--At the novel's beginning Jem and Scout have a difficult time seeing their father, a lawyer, as anything but a decrepit old man who is good for nothing (at least nothing that they're interested in). As the novel progresses, the children begin to identify with their father more and to learn things from his past that prove that he was young once, is not as old and useless as they thought, and that he is actually a town hero. Use examples such as Atticus's shooting the rabid dog, his defense of Tom Robinson, and his buying the children air rifles (after all, he remembers when he was a child and had a gun).
2. Jem--Jem is profoundly affected by watching his father during Tom Robinson's trial. He is a child before the trial begins and a young man who has lost his innocence after the trial. Lee seems to imply that Jem might be a lawyer when he grows up. Jem's discussion with Atticus about rape being a capital offense and the makeup of juries demonstrates that he is now thinking of weighty adult topics. As the reader witnesses Jem's maturation, he gets a picture of what Atticus might have been as a child.
3. Scout--the novel's precocious narrator learns much from her father and also from watching her father's opponent, Mr. Gilmer. She realizes that people do not become cynical or wise overnight and that there must be events from their past that cause them to view the world in the way that they do. She talks to Dolphus Raymond outside the courthouse, and she listens to Miss Maudie's description of her father's reputation.
All of these examples demonstrate that children--even those who become lawyers--lose their innocence at some point and must begin to view the world as adults.