One simile author Harper Lee uses in To Kill a Mockingbird to mark Jem's progress in maturity as he grows up is found in Chapter 7 when he relays to Scout what happened the night he went back to the Radleys' property to retrieve his pants in the wee hours of the morning.
After that night, Scout noted that Jem hardly spoke a word. It is not until after school starts up again and they begin finding more items in the knothole of the oak tree that Jem starts speaking again. While crossing the schoolyard on their way home from school one day in the afternoon, Jem preludes his story of the night's events by saying, "There's something I didn't tell you ... [a]bout that night." When Scout interrupts, saying, "You've never told me anything about that night," Lee uses a simile to describe Jem's annoyed yet pensive actions:
Jem waved my words away as if fanning gnats. He was silent for a while, then he said, ... . (Ch. 7)
Next, Jem describes how he found his pants laying on the fence, neatly folded, and mended. Waving away Scout's words indicates he wants her to be silent because what he is about to explain is very difficult for him. His silence further shows that he has been thinking long and hard about what he is about to explain. Jem's thoughtfulness indicates he has been thinking long and hard about Arthur (Boo) Radley and beginning to think that their judgements of him may be completely wrong, which indicates Jem is growing up.
Another good simile is spoken by Jem to Miss Maudie the day after Tom Robinson's trial. Jem feels angered by the jury's guilty verdict and disillusioned by Maycomb because he now sees Maycomb's people as hateful, racist people, whereas he once saw them as good and decent folks. Jem explains his feelings of disillusionment in the following simile:
It's like bein' a caterpillar in a cocoon, that's what it is ... . Like somethin' asleep wrapped up in a warm place. I always thought Maycomb folks were the best folks in the world, least that's what they seemed like. (Ch. 22)
Jem's feelings of disillusionment show that he is maturing beyond the innocent boy he used to be. As a boy, he only had a perception of goodness and innocence, but now, the things he has witnessed has allowed him to see the evil that exists in the world, something he could only see if he is maturing.