This is way off my usual beaten path, but here is Hillary Kelly in the New Republic:
People have the wrong idea about Lolita, and Lolita. Today, the name is widely used as a synonym for a sexually precocious young girl. But the most important fact of the novel is that Lolita is a 12-year-old girl....This makes the oversexed, hyper-titillating cover art that has been repeatedly slapped on Lolita incredibly bizarre—not to mention disturbing. We aren't meant to find Lolita sexy. We shouldn't find Lolita sexy. Nabokov himself said that readers were "misled" by the book's repuation "into assuming this was going to be a lewd book." I'm not so naïve as to imagine book covers always faithfully replicate the literary intentions of their authors. But Lolita covers aren't simply exaggerated or oversimplified representations. They're downright creepy.
Huh. I didn't know that. But there's a good reason for this: I've never read the book. Like a lot of people, however, I have seen the movie. And in the movie, Sue Lyon plays a teenage Lolita. So I always figured Lolita was indeed a high-school age girl. I don't know if Stanley Kubrick made this decision for artistic reasons or—ah, wait. Sure enough, the ever-helpful Wikipedia informs me that "Lolita's age was raised from twelve to early teens in the film to meet the MPAA standards. As such, Sue Lyon was chosen for the title role partly due to her more mature appearance."
Anyway, I wonder if this is the wellspring of much of the common confusion? I'll bet a whole lot more people have seen the movie than ever read the book.