As a sign of southern respect, Calpurnia begins to call Jem "Mister Jem." It is a rite of passage for Jem because he is growing up and becoming more of an adult. It is also a sign of respect Calpurnia, a black woman, shows to her young “employer.” Jem is now twelve years old in the story and beginning to truly understand more and more of the adult world. He has a newfound respect for his father and wants to emulate his honesty and character. Unfortunately, as Jem grows in the story, he begins to leave Scout behind and even nags her to act more like a girl and not to speak to him when he is with his pals at school. Scout even notices that Jem prefers to be by himself instead of playing games with her. She also says he is moody. When Atticus loses the Tom Robinson case, Jem’s childhood idealism is shattered by the realities of an adult, racist society. However, it is the same grown-up Jem who protects Scout and risks his own life when Bob Ewell attacks them.
“Mister” Jem shows how he is coming of age in the adult world and, therefore, is leaving his childhood behind.
Jem has just turned 12 years old at the beginning of Part II (Chapter 12). Although he is not yet a teenager, turning 12 seems pretty important to everyone in the family except Scout. Calpurnia begins calling him "Mister Jem," and both Cal and Atticus try to explain to Scout that Jem is "growing." He no longer wants to spend his spare time with Scout, and he warns her that it's about "time you started bein' a girl and acting right!" Jem is feeling the first signs of puberty, and it will be a difficult time for both Scout and Jem, whose "maddening superiority" eventually led to a fistfight between the two.