Mark Twain had already used Huckleberry Finn as a character in The Adventures of Tom Sawyer, which is a children's book narrated by an omniscient adult. It is an interesting choice, therefore, to have The Adventures of Huckleberry Finn, a much more ambitious novel for people of all ages, narrated by a child.

Huck's perspective, however, is a wide-ranging one. He is innocent, and ignorant, but not stupid. He provides a foil for Jim, who is generally wiser and more sensible but less educated, just as Huck himself was a foil for Tom in the earlier book. However, his own lack of book-learning soon becomes apparent to the reader in his naïve explanations.

Huck's age allows for a combination of innocence and shrewdness which allows the author to move quickly from comedy to something much more sinister. He is impressed by Emmeline Grangerford's terrible poetry and, indeed, by the polish and elegance of the Grangerford family in general. However, he can also see quite clearly the absurdity and evil of their bloodthirsty feud with the Shepherdsons.

Immediately after this episode, Huck and Jim are joined on the river by the Duke and the King. While Jim is taken in by these two, Huck has seen enough of adult hypocrisy to spot them as frauds easily enough. However, he pretends to believe them, and the four travel together for some time. Huck's behavior here might seem cowardly in an adult, but he has never had any alternative except to accommodate himself to the wickedness and folly of the adult world. All he can do is to comment sardonically on what he sees for the benefit of the reader.

One of the reasons that Mark Twain used Huckleberry Finn, a 13-year old boy, as the narrator of his story might be that Huck has a lack of knowledge about the world that an adult would have. He also has to fight against people who want what they believe is best for him; as an adult, he'd have the opportunity to leave and live the way he wants.

When Twain chose to write about a child, he chose a different set of problems and perspectives than he'd have written about otherwise. Adults also usually have some kind of connections that keep them tied to a family or a town. They have responsibilities. Huck doesn't and he's able to choose how he wants to live and leave with Jim.

Another reason that Twain chose to write about a 13-year old boy is that Huckleberry Finn was based on a real boy who he says was named Finn. Since the character is based on a real person, it would make sense for the age to be close to that person's age as well. When he speaks of Tom Sawyer and Huck Finn, Twain says, "I rather enjoyed writing them. The characters were no creations of my own. I simply sketched them from life. I knew both those boys so well that it was easy to write what they did and said. I've a sort of fondness for 'em anyway."

Authors of works of fiction frequently use a character in their story as the narrator.  Narration helps the reader to fill in gaps in the story that might otherwise leave the reader with many questions regarding motives and thoughts about situations and other characters.  Especially in a story wherein the main characters, Huck and Jim, are in a remote or isolated setting, for example, a raft going down a river, first-person narration is a natural method of describing the action and the character's observations.  First-person narration also has the benefit of allowing the author to employ dialects, slang, accents, et cetera, in a more personal manner than when used in third-person descriptions.

Twain did not have to use first-person narration in his story.  That he chose to do so was consistent with his writing style and the way he liked to tell stories.

Why does Mark Twain use a young boy as the narrator in The Adventures of Huckleberry Finn?

Twain's purpose in The Adventures of Huckleberry Finn is to expose moral hypocrisy in "good" society, and he used attitudes in the South toward slavery before the Civil War as his touchstone. In order to effectively satirize the cruel social norms about slavery that passed as righteous and civilized in the Old South, Twain needed a naive character. That character is Huckleberry Finn, a marginal, untamed figure in his society who cannot quite learn how to behave or internalize the rules, but who nevertheless believes unquestioningly in the "morality" he has been taught.

By showing how innocently Huck believes he is a bad person and a sinner for protecting an escaping slave, the novel reveals the sick, upside-down morality of slavery and, by extension, any exploitation of one human by another. Of course, we as readers know that Huck is acting morally and courageously as he befriends Jim, even if Huck does not.

At a pivotal point in the novel, Huck assumes responsibility for himself and makes a moral decision that he believes will send him to hell. Nevertheless, he follows the dictates of his heart. In some of the most moving lines of the novel, we as readers learn the essence of true morality from this innocent, ostracized boy who is willing to sacrifice not only his body but, as he believes, his eternal soul to protect a person he loves:

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 [a letter exposing Jim] 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.

There are several reasons why Twain chooses a young boy to tell the story in The Adventures of Huckleberry Finn.  First of all, Huck originally showed up in the story, The Adventures of Tom Sawyer as Tom’s sidekick.  Twain took the character, Huck, and developed a more symbolic picaresque novel that contained satire and the rite of passage of a young boy.  Twain wanted The Adventures of Huckleberry Finn to be more than just a children’s story and set out to create the perfect individual to tell the tale.

One reason Huck is so young in the novel is because Huck is still impressionable.  Even though he understands the values and beliefs of society, he is willing to go against those conventions.  He is still in the stage of life where he can be an individual and not adhere to society’s mores.  He has led an unusual life in his short existence as well.  He is an orphan who has had to provide for himself and learn to survive.  He doesn’t like the ties society places on him like going to school or church.  He would rather be on his own doing what he wants. 

Huck also doesn’t have any restrictions placed on him.  Because he is young, he doesn’t have responsibilities like a job or family to get in the way of his self-discovery.  He is free to do what he wishes without duties bogging him down. 

Foremost, Huck is wise and able to eventually see society for what it really is.  By telling the story through the eyes of a young boy, we are able to see him progress and mature morally as he learns about life.  The use of a young narrator allows the reader to see how the character grows and matures because of the obstacles and hardships he faces, because of the experiences he has in life, and because of the mature or immature decisions he makes.  Through Huck, Mark Twain is providing for the reader a whole new life seen through the eyes of a 13-year-old boy as he first experiences the real world.

Why did Twain choose Huck, a thirteen-year-old, as the protagonist in The Adventures of Huckleberry Finn?

The great critic Alfred Kazin sheds some light on this question in an essay used as an afterword in some editions of the novel. Kazin posits that this novel was intended to follow Twain's other Mississippi novel, The Adventures of Tom Sawyer. 

Huckleberry Finn was inferred to be just a sequel toTom Sawyer.

Regarding the earlier book, Kazin quotes Twain as saying: 

"Part of my plan has been to try to pleasantly remind adults of what they once were themselves, and of how they felt and thought and talked, and what queer enterprises they sometimes engaged in."

Though the subject of the story of the later novel became more serious than anything in the Tom Sawyer story, the intention was still to fulfill the goal of the quote above. 

We can see this intention borne out with the appearance of Tom Sawyer at the end of Huck's story. When the story is poised to become truly serious, with Jim captured and enslaved, Tom Sawyer arrives full of antics, humor, and wit, effectively saving the novel from its serious impulses. 

The above explanation offers, perhaps, the central reason that Twain chose to use a boy as his protagonist - his intention was to remind adults of what childhood was like. 

Regarding the themes of the work, Huck Finn's age works in favor of an honest contemplation of the disconnections between accepted social norms and what is morally right in a given situation. Huck is in a place, developmentally, where he is divided between his moral instincts and his moral instruction.

As a child, Huck is involved in the process of establishing his personal moral code. This makes Huck Finn a good choice for the protagonist of a novel examining the morality of slavery (in a way) and the morality of loyalty which troubles the boy repeatedly.

Although he does not acknowledge it as such, it is Huck's development of a higher standard than that of contemporary mores that enables him to partially overcome the dictates of his conscience and act the part of a "nigger-stealer."

Why does Mark Twain use a young boy as the main character in 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.