A good question, and I'm not entirely sure myself. Who reads it most is probably the best criteria, but is flawed, at least in this case. Most people probably read Huck Finn somewhere around 15-18, but it's also often assigned reading. This metric only really works when it's voluntary reading, otherwise you're selecting the "appropriate" age by what has been pre-established by a third party, which is a measurement I don't agree with in the slightest; simply because everyone SAYS that's the age that one should read the book doesn't mean it is.
Who the author intended the target audience to be is tricky, too; Twain very well could have been writing to a child/young adult audience as a sort of teaching novel, but this is only really valid in the contemporary setting if so, and there's a good argument against it (though don't quote me on that, the details are kind of fuzzy on this front.) Then of course there's the intentional fallacy and all that goes along with that, which is complicated in how much you buy into it, but that's a different subject for a different time.
Who it is marketed towards now has sort of already been dealt with: just because it is being marketed at an audience doesn't mean that it's the appropriate audience.
The best answer I can give is still no good: What audience is best able to understand the text. I don't really think that Twain's writing is all that accessible to a juvenile audience; I recall reading it at 16 and thinking it was crap, and re-reading it at 21 and finding a lot more value in it, for instance. Of course, there will be people who can understand the text at 15 or 16, which complicates the matter, but in all honesty those people are few and far between. Still, I wouldn't trust the typical teenager to have easy access to the book, and there is the fact that most often when they pick up the book they only do so because they have to, not because they want to.
I'll have to think more on this to really give a coherent answer, though.
