How does Jem change in "To Kill a Mockingbird"?
in chapters 12-15 I think
Jem was physically changing, as he turned 12 in chapter 12. But more importantly, he was beginning to think of himself as an adult as well. Scout sees his almost "overnight" changes and describes them in her narration.
"Overnight, it seemed, Jem had acquired an alien set of values and was trying to impose them on me: several times he went so far as to tell me what to do."
In this section he even tells her that she needs to act more like a girl--the very opposite of what he used to say--that she needed to stop acting like such a girl. He also has developed a lofty sense of wisdom, too. He reads the paper and doesn't want Scout to bother him.
Then in chapter 14, Jem "broke the remaining code of [their] childhood" by going to get Atticus when Dill showed up under Scout's bed. He thinks more like an adult than he used to, and he's trying to act more like one as well. He went to tell Atticus because he knew that Dill's family would be very worried. That's an adult's concern, not a kid's.
Jem was growing older and believed that he was more of an adult. He should not play games and he also became more responsible. Jem started to become more like an adult and more of the characteristics that his father would produce. This surprised Scout as she was used to seeing a carefree Jem.
I actually did an essay on this.
I don't know about 12-15, but i do know he changed as a person throughout the entire play. He became more responsible and caring and aware of his little sister. He turned into a man.