In To Kill a Mockingbird, how does Jem break what Scout calls "the remaining code of our childhood?"

There is definitely the indication here that Jem, being
older, is progressing into adulthood. Just before they discover Dill, he and Scout get
into a fight. Then, moments later, he does the responsible thing by telling Atticus that
Dill has run away. All three children mature throughout the novel, but Scout and Dill
still have an innocent and naïve curiosity about the world. “Breaking the code,” in this
instance marks a separation. Jem enters the adult world by telling Atticus. Scout and
Dill would have preferred to stay in their own world and kept it secret. This
separation, Jem/Scout and Dill, marks the moment the break of that remaining
code.