Just finished reading Spider Robinson/RJ Heinlein's YA novel "Variable Star" (pub 9/2006). The story had some good ideas, like Relativist who make starship engines run near light speed by thinking, telepathic twins for instant communication between star systems, intelligent design by less-than-omniscent beings.
But do YA readers need the f**k word used in the book to express adolescent frustration? Twain didn't use it, neither did CS Lewis and most other YA authors of previous generations.
Robinson's language selections for his Joel Johnston 18-year old hero seem aimed generally at a slightly younger group of male readers. Expressing the angry and frustrated side of human emotions without resorting to strings of epithets may be difficult, but precisely because of the nature of the YA audience, it should be a writer's goal. If the dialogue is only a reflection of the way the writer believes his audience actually speaks, that still isn't an adequate reason for limiting vocabulary.
Even Christian writers might sometime need to include a cussword in story dialogue, but a constant stream of vile invective is never necessary.