Tom Sawyer: Maturity, 3 examples

Tom takes the punishment for Becky.

“Rebecca Thatcher” [Tom glanced at her face—it was white with terror]—“did you tear—no, look me in the face” [her hands rose in appeal]—“did you tear this book?”
A thought shot like lightning through Tom’s brain. He sprang to his feet and shouted—“I done it!”

Tom kept Becky safe....

"Tom knew their ways, and the danger of this sort of conduct. He seized Becky's hand and hurried her into the first corridor."

In the conclusion:

"It being strictly a history of a boy, it must stop here; the story could not go much farther without becoming a history of a man"