I think it's interesting how many people are determining whether a book is appropriate or not based on the word choice, rather than the conceptual content. RWTB has "bad" words in it, but tells a good story--in many respects a morally admirable story. The latter ought to be infinitely more significant than the former.
The Bible contains a whole host of words that would get you in trouble if you said them in a middle school classroom--but they're unlikely to ban it from the same library.
With regard to the age at which people ought to be able to learn that people have different standards for what language is offensive, and that some people will use words that others don't like--it's far, far before 14. Somewhere between three and five years of age parents ought to be teaching their kids that there are some words that they don't like to use because they consider them rude on inappropriate. They should be taught that other people or other families might use these words--or that our family might use some words that others might find offensive. We shouldn't look down on people who use words we don't use, or try to offend people who are offended by some words we use--but it's the ideas behind the words that we should be evaluating.
Kid can read the book, see some words he thinks are rude, comment to himself that he doesn't like the language and won't use it himself, and take what he can from the good lessons in RWTB.