In To Kill a Mockingbird, what does Scout mean when she states, “It was then, I suppose that Jem and I first began to part company,” and what...

Scout realizes that Jem is getting older and they no longer think alike when he risks everything to retrieve the pants he left at the Radley house.

Scout makes her comment about parting company with Jem after the pants incident.  Jem, Dill, and Scout tried to pass a note to Boo Radley that would make him come out of his house.  Boo was a neighborhood boogeyman, but Dill was obsessed with him.  He felt sorry for him and thought he would feel better if he got to know them.

Jem accidentally leaves his pants at the Radley place, and tells his father that he lost them playing strip poker.  He sneaks back at night to get them, because he is afraid that Atticus will punish him when he finds out.  Jem explains to Scout that it is not the punishment he fears; it is Atticus’s reaction.

He blew out his breath patiently. “I—it’s like this, Scout,” he muttered. “Atticus ain’t ever whipped me since I can remember. I wanta keep it that way.” (Ch. 6)

Scout tells Jem not to risk his life just so that he won’t get spanked, but to Jem it is about Atticus’s opinion of him.  He doesn’t want Atticus to know that he did something he wasn’t supposed to.  He wants his father to think of him as more grown up.

Scout realizes that she and her brother have different priorities.  She would not have cared if Atticus found out.  She was afraid to go back to the Radley place and be shot by Nathan Radley.  Jem was willing to risk it to keep Atticus in the dark.

It turns out that Boo Radley understood.  To keep Jem from getting in trouble, he sews up the pants and leaves them for Jem to find.  Jem's reaction to and understanding of Boo Radley also demonstrates how he is more mature than Scout.  She is still afraid of him.