I teach 11th grade English, and have taught The Adventures of Huckleberry Finn for years now. Kids at that age do struggle with it, for a couple reasons. First of all, the dialect is very difficult to understand, and kids have a hard time working their way through it and comprehending what is being said and done, because it is in a slang and style that they are entirely unused to. The second reason that kids struggle with it is that its format isn't very traditional; they are used to stories with clear-cut beginnings, middles, ends, and points or lessons to be learned throughout. Huck's rather random and haphazard journey through America in the 1800's is hard to follow and find a point to. I have kids ask, "What was the point of this book?" So, that is a struggle for that age group. With some more maturity, both the dialect and style might go easier for students.
All of that being said, most of my kids really enjoy the book for a couple reasons. First of all, it's fun. Huck is a hugely likable character, is super clever and funny, and has some pretty fantastic adventures that are interesting and suspenseful. Students like him, and like the fun stories. Secondly, because it IS fun and there isn't a lot of in-depth haranging over theme and deeper meaning, it is a much more laid-back and pleasurable experience for them. They actually enjoy it a lot, and have a postive experience.
A lot, however, depends on the teacher and how they teach it. When I teach it, I don't have kids read it straight through; instead, I pick out the best segments of the novel, and through a lot of explanation, activies, games, and hands-on instruction, we just have fun with it. We bring out fortune cookies to talk about Jim's predictions with the oxen hairball, we draw pictures of Huck's escape from his father, and we laugh and moan over Tom's ridiculous exploits to make Jim's escape more "dramatic." I don't take it super seriously, help them a lot with the dialogue, and make sure the kids have fun with it.
So, it can be taught to 11th graders, but carefully, with a lot of help and activity. I hope that those thoughts helped; good luck!