Why do you think Mark Twain uses a young boy as the main character and narrator of The Adventures of Huckleberry Finn?

Huckleberry Finn is an appropriate and complex character who approximates many of the quintessential features of the United States in the mid-19th-century era when Mark Twain lived and about which he was writing. Featuring a boy in the protagonist role gave Twain considerable flexibility in the plotting, exposition, and satirical humor that are notable aspects of this work. Huck’s physical journey down the river has become a classic—some would say, the classic—statement of personal growth and the development of the US through westward frontier expansion. He can also embody some ethical contradictions that Twain wanted to put forward about slavery.
If we think about the pairing of Huck with Jim, an escaped slave, we can see that an adult male could not occupy Huck’s role. As a poor boy who has suffered at his father’s hands, Huck empathizes with Jim in a way that an adult probably would not admit to feeling. From Jim’s perspective, he can reasonably believe, if not absolutely trust, that the boy will not turn him in. Huck’s innocence extends to his lack of full understanding of what slavery is and his acceptance/belief that helping Jim might mean he'll go to hell. This decision has often been taken for a metaphor of the United States moving toward emancipation.
Huck is a good-hearted person, if not completely honest, but he is not clever like his friend Tom Sawyer. Huck is more of an Everyman character, in whose good nature Americans could see themselves. He much resembles the heroes of Charles Dickens, such as Pip, who negotiate a variety of social roles and depend upon the goodness of adults to help them realize their goals and grow into decent men.