What is the resolution of Huckleberry Finn?

Huckleberry Finn was written for an adult readership, but Twain's style uses a relatively simple vocabulary and format.

There are several reading level scales in use, some based on age some on grade. All are dependent on the native language of the reader.

After all that, it would appear that readers as early as Grade 4 (+9 years) could enjoy the book although some of the social and historic topics might require explanation. The irony and sarcasm might escape many readers until they are significantly older and more bitter.