Labeling Books: Are YA and NA Genres Or Reading Levels?

Some consider YA and NA a genres, while others call them reading levels. The latter often leads to criticism of adults who read YA. So what exactly do the labels YA and NA mean?

Genre: If YA and NA are genres then they are reading categories. Horror, romance, science fiction, fantasy, ect... are all genres. Many libraries and book stores have separate sections for YA and NA novels, which would seem to support the idea that YA and NA are genres.
Reading Level: A reading level is a scale used to give information on how old a person needs to be to either comprehend the writing in a particular book or how old a person needs to be for the content of a particular book to be appropriate. Reading levels are like movie ratings, they help direct a particular audience to a book based on their age. Considering many YA books are written with young adults as the intended reader, this idea makes some sense.
However, as an avid reader and writer of both YA and NA novels I wonder if the labels YA and NA are neither a genre nor a reading level.
So, if YA and NA aren't genres or a reading levels then what are they?
Point of View: I think the best answer is that YA and NA are simply points of view. The type of story will determine the genre. For example a YA Paranormal Romance is a story in the Paranormal Romance genre told through a young adult's point of view. As for reading level, the content of the story will determine that. Because YA stories are told through the eyes of a young adult character many of them will be appropriate for a group of readers the same age as the protagonist. But, that doesn't mean you can't write a story appropriate for a younger or older audience with a main character who's in their teens.
So if YA and NA are points of view, does that mean anyone can read them? YES, but they are written for a particular audience. YA books are written for people who want to see the world through a teenager's perspective. Anyone, of any age can enjoy reading about a character who is dealing with coming of age problems, like falling in love for the first time. The same can be said for NA books. These books are written for people who want to read about a character dealing with the types of things one deals with in their twenties, like becoming independent.
So there you have it, those are my thoughts on what the labels YA and NA mean. What do you think?