What changes does Calpurnia make in the way she addresses twelve-year-old Jem?

Now that Jem has reached the age of twelve, people start acting differently towards him. It's notable, for instance, that Miss Maudie bakes Jem a bigger cake instead of the smaller ones she's prepared for Scout and Dill. Although, as Scout herself notes, Jem appears to have lost his appetite of late, in addition to becoming moody, inconsistent, and difficult to live with.
Things are changing for Jem on the home front too. Calpurnia starts calling him Mister Jem, which slightly confuses Scout. She doesn't think he's old enough to be addressed as such. But Calpurnia is a lot older and wiser than Scout; she knows that Jem is on the cusp of adulthood, wanting to go off all by himself and do his own thing. Scout still may not fully understand, but at least Calpurnia has gone some way towards explaining Jem's strange recent behavior.