Question. And thx this was helpful. I'm knee-deep in the dilemma you're speaking of. My MC is 13 (I aged her up from 12 to 13) but the main supporting chars. are 13, 17, 17. There's some graphic violence also. Genre: contemporary/horror with supernatural elements. Think: The Lost Boys with a 13 y/o protagonist. themes and writing style, as of now, fit YA, both for the age of the reader and for the graphic nature. But again I have a very young MC. *help:)
Obviously, I'm commenting sight unseen, but not a lot about this sounds MG, especially the graphic violence. I’d age the thirteen and fourteen year olds up to fifteen and target YA, or be prepared to hear similar feedback in submission. Best of luck!