At the beginning of Part Two of To Kill a Mockingbird, Jem has just turned 12 years old.
He was difficult to live with, inconsistent, moody.
He was growing up and reaching the first stages of puberty. He no longer wanted to spend as much time with Scout and, as Calpurnia explained,
"He's gonna want to be off to himself a lot now, doin' whatever boys do..."
Jem had also
... acquired an alien set of values and was trying to impose them on me... In addition to Jem's newly acquired characteristics, he had acquired a maddening air of wisdom.
He was also feeling more like a man. Although neither of the children particularly care for their Aunt Alexandra, Jem tries harder to get along with her or, at least stay out of her way. When Atticus and Alexandra argue about Calpurnia's place in the Finch home, Jem blames Scout. He orders her "not to antagonize Aunty, hear?" Scout accuses Jem of being bossy, but he tells her that they don't need to be "worrying" Atticus with the upcoming trial taking up so much of his time. Believing that he is now an adult himself, Jem tells Scout that
"... you can't hold something in your mind but a little while... It's different with grown folks. We--
With that, a brawl began between the two, and they were both children again. With the arrival of Dill, Jem allowed Scout to reacquaint herself with her "permanent fiance," preferring to spend his evenings alone in his bedroom with "a stack of football magazines." In the later chapters, Jem proudly displays his first evidence of chest hair, and announces that he plans to go out for the football team.