In Chapter 12 of To Kill a Mockingbird, what changes does Calpurnia make in the way she addresses 12 year old Jem?

In Chapter Twelve, Calpurnia refers to Mister Jem, rather than Jem, which confuses Scout. Calpurnia changes the way in which she refers to him because in her eyes, Jem is becoming a man.

“Don’t you fret too much over Mister Jem—” she began. “Mister Jem?” “Yeah, he’s just about Mister Jem now.”