In what way does Jem break the one remaining code of childhood in Lee's To Kill a Mockingbird?

Jem broke one of the last remaining codes of childhood when he told Atticus about Dill running away from home, showing that he was thinking like an adult because he knew that Dill’s mother would be worried.

When they first met Dill, Scout and Jem found out that his father was not in the picture.  Dill was shipped off to his Aunt Rachel every summer, which was good for Scout and Jem, but also made him feel unwanted.  The first time Dill did not come, it was because his mother remarried.  Dill felt like he finally had found a father replacement.

Scout found Dill hiding under her bed.  He wove them a story about being chained in the basement by his stepfather and fed peas through the ventilator by a passing farmer until he ran away and joined a small animal show.  It was a typical Dill whopper, designed to hide the pain that Dill really felt about being ignored by his stepfather.  Scout wasn’t going to tell anyone he was there, but Jem was older and realized that their father had to be told.

“You oughta let your mother know where you are,” said Jem. “You oughta let her know you’re here…”

Dill’s eyes flickered at Jem, and Jem looked at the floor. Then he rose and broke the remaining code of our childhood. He went out of the room and down the hall.

“Atticus,” his voice was distant, “can you come here a minute, sir?” (Ch. 14) 

Although Scout said that Jem broke the childhood code in telling on Dill, and they were mad at Jem at first, she told Dill not to stay mad at him.  Jem did what he thought he had to do.  Dill told her that he wasn’t mad.  He understood that Jem told on Dill for his own good.  When they were alone, Scout asked Dill why he ran away. 

By this time I was, but lazily so. “Why’d you do it?”

No answer. “I said why’d you run off? Was he really hateful like you said?”

“Naw…” (Ch. 14)

Dill says that he ran away (by stealing money from his mother’s purse and taking a train) because he felt that he was not wanted.  His mother and stepfather would go off into the next room together and leave him alone.  His stepfather did not spend any time with him.  He felt lonely, and the fantasy of a stepfather did not play out.

This incident of Jem breaking the childhood “code” demonstrates again the distance in maturity between Scout and Jem.  Scout wants to keep Dill’s secret in solidarity, but Jem realizes that Dill’s mother is worried about him.  After Jem turns Dill in, Scout understands why, but she still resents it.