Explain how breaking this "code" shows jem's maturation from childhood to a teenager.

As a result of growing up and maturing, Jem now understood that there were some things that couldn't be kept secret. Yes, he cared about Dill and was his friend, but he also understood that Dill's actions affected more people than Dill himself. His decision to tell Atticus was one of maturity, and one that may not have been decided the same way not so long before.