Various people have questioned some of the things I said in my previous post as being accurate. While I do stand by my assumption that the asker's 65k fantasy novel is YA just based on its size unless it was specifically written for a younger crowd, I am not an expert on children's literature.
While I have dealt with children's literature in the publishing field to a small extent, most of my knowledge of what grades are reading what comes from a course I'm taking on children's literature in graduate school, by a professor who's written many children's books and runs various children's poetry and children's literature festivals in New York City. There were many things I honestly didn't know before taking this course.
I admit that the line between middle school, young adult, and adult literature gets kinda hazy in age range, and that a lot of it has to do with the school and culture you grew up in. An ESL student is obviously probably going to be a step behind in literature. As I mentioned, I was the first person in my class to read an "adult" book, which was not available in my school library and I had to get at the public library in town. This was mainly because it was Jurassic Park and the movie was coming out sooon, so I got the book, and I'll readily admit that I did not understand half of it. (Also, the first half of it doesn't make a lot of sense) The following month I read The Andromeda Strain and also didn't understand a word, except that a disease was eating a plane or something. It wasn't until I hit Sphere that I started "getting it," and breezed through The Great Train Robbery, which was easier because it was historical fiction instead of science-based fiction. Obviously, one of the reasons I've ended up in publishing is because I love to read. I read 1-3 books a week.
I distinctly remember that we stopped reading YA and started reading "literature" in eight grade, when I switched to a private school that had already been doing it for two grades. Shakespeare, Maya Angelou, and Philip Roth.
I'm sure this varies from school to school, so I wouldn't doubt that there are 15-year-olds out there who are reading YA. There are adults who've realized there is some terrific literature being put out in YA and are eating it up.
Anyway, the point is: I don't know everything.