How does Huck mature?

Huck matures through facing the moral dilemma of whether he should help Jim escape slavery. All his life, Huck's society has taught him the warped morality that helping a slave escape is a great evil. Nevertheless, he does help Jim. For a long time, he doesn't think about it much. He doesn't fully wrestle with the moral issues. But when he does, he learns to think for himself and make his own moral decisions. Although he fears he will be condemned to hell for it, he trusts his own inner voice. This voice tells him that the friendship with Jim and the many ways Jim has shown himself to be a loyal, worthy, and exemplary, if imperfect, human being are far more important to him than the racism he has been taught. For Huck, the quality of Jim's character counts for far more than the color of his skin.
We mature into adulthood as we become capable of making moral decisions for ourselves rather than simply relying on the authority of our elders. In one of the most famous and moving passages in the novel, Huck thinks very hard just as he is about to reveal Jim's whereabouts:
And [I] got to thinking over our trip down the river; and I see Jim before me all the time: in the day and in the night-time, sometimes moonlight, sometimes storms, and we a-floating along, talking and singing and laughing. But somehow I couldn't seem to strike no places to harden me against him, but only the other kind. I'd see him standing my watch on top of his'n, 'stead of calling me, so I could go on sleeping; and see him how glad he was when I come back out of the fog; and when I come to him again in the swamp, up there where the feud was; and such-like times; and would always call me honey, and pet me, and do everything he could think of for me, and how good he always was; and at last I struck the time I saved him by telling the men we had smallpox aboard, and he was so grateful, and said I was the best friend old Jim ever had in the world, and the only one he's got now; and then I happened to look around and see that paper.
It was a close place. I took it up, and held it in my hand. I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. I studied a minute, sort of holding my breath, and then says to myself:
“All right, then, I'll go to hell”—and tore it up.