That's over-simplifying. These days adult readership for YA seems to have a significant impact on the market, to the extent that publishers know they're not just selling to an age group. YA is about perspective. These books are told from the point of view of teen protagonists, and deal with issues like being initiated into the adult world or addressing problems posed against the protagonist by the adult world (according to one article in The English Journal, which refers to YA as a genre). The way they're written, they continue to have appeal across age-levels, and they vary so much in style of writing and sophistication that I think it makes less sense to say it is targeted at a certain reading level than to simply view it as a group of shared characteristics in terms of character, theme, aesthetic, and the like. And that starts to look a whole lot more like a genre (which is why people refer to it that way).
And yet when I hear that genre name, I know something about it. I think of it more like a family within which genus and species are contained.
Yes, when you hear the target audience, you know the target audience. You know nothing about the setting, plot...
There's nothing in the definition of genre that limits it to setting, plot, etc. For example: gen·re [ˈZHänrə] NOUN a category of artistic composition, as in music or literature, characterized by similarities in form, style, or subject matter. (Oxford Dictionaries) Similarities in form (depending on how you want to characterize that) and subject matter (themes addressed and like) fit into my explanation above as to why YA looks a lot more like a genre than simply an age range. There is also quite a lot of similarity in style across a lot of YA, though not all of it (then again, SF, F, Horror, etc. don't all share the same style either).
