The brave independence of spirit and of moral thinking expressed in Huck's narrative suggests that maturity of years is less important to spiritual or emotional maturity than honesty is. Huck is capable of being honest, even when this means admitting his uncertainties. Many others, adults especially, are absolutely sure of themselves yet become subject to fraud, deception, and general folly.
Consider the time Twain was writing. First of all, he sucks the readers in with Huck Finn. We already know Huck from the completely safe Adventures of Tom Sawyer. The characterization of Huck in that book is also perfect for the story of a boy and a slave running away, since Huck is already a social outcast.
This novel was also revolutionary for its use of dialect, and having Huck as the narrator allows both dialogue and description to come from a low class, uneducated, small town Missouri boy. The reader gets the full experience of the language and the perspective that comes from Huck, and not some omniscient narrator.
Linguistically, Adventures of Huckleberry Finn has been studied by many authors and language experts for its amazingly well done dialect. Shelley Fisher Fishkin wrote a pretty comprehensive book called "Was Huck Black?" that argues that Huck's dialect was inspired by and drawn from African American dialects. This book is really good at exploring how slave dialect influenced "Huck Finn" and white slave-holding culture in general. Very interesting scholarship on Twain!
Having Huck as the narrator was a revolutionary step in American literature. The first sentence, "You don't know "bout me unless you have read a book by the name of. . ."immediately lets the reader know that this book is going to be realistic. The grammar and the spelling are not traditional and descriptive as many romantic books which had proceeded it. Having a 13-year-old boy as a narrator means that Twain is free to teach us lessons as Huck learns them. We see the action through his eyes but interpret the action through our own. This allows the reader a unique perspective. Because the narrator is so young, he can be a rebel without being threatening. Twain is free to explore many controversial issues but those issues are seen through a young boy's eyes and are not a threatening as they would be if seen through an older narrator. Huck gets away with things an adult narrator would never even attempt. In addition, he can question society in a way no adult would and his thoughts somehow become our thoughts. Thus, it allows Twain great flexibility to explore society in ways that would be impossible any other way.