Here I will dare to respectfully disagree with the wonderful and published XimeraGrey, whose opinions on publishing issues I’ve come to respect. I only dare because I had the opportunity to discuss topics like this with some trad-published YA authors.
To summarize, trad is very sensitive to classification factors such as protagonist age when it comes to certain categories, most especially YA. One published YA author put it simply: for a debut YA, your protag has to be 16, and that’s it. Another said 15 and 17 are possible (she was being nice, because my protag happened to be 14), but agents and publishers are looking for 16. I found quite a bit of corroborating evidence in agent blogs. I’ve since aged my protag to 15
The readership age bracket for the trad “YA” category is 13-18 year olds (not 18-22 as another poster suggested; technically 18-22 is ‘New Adult’, but that’s a whole different kettle of soft porn … I mean fish). Like pre-teens, younger YA readers like to ‘read up’. They want to read about how characters deal with issues they already sense they’ll face with when they reach that age - if not before. This ‘read up’ effect peters out around age 17, because by then, they know it all, right? Would a 15 year-old want to read about a 12 year-old? Generally not.
YA is also about tone and theme and topic. YA is self-focussed (some would say self-obsessed), relationship-oriented, personal and social issues-exploring, etc. All these coming-of-age things. For a twelve year-old protag to be that inward-looking would probably read as unnatural. To a YA reader, the outward-focussed, drier tone of a natural child protagonist tends to be less interesting.
So if you’re trying the traditional route, I think you got it right with “I wanted it to be a coming of age for adults since the genre is horror and it has scary, a bit gory, and explicit content,” It sounds like something a publisher would classify as Adult. Note: Adult us much less sensitive to protag age - pretty much anything goes. It’s all about the story.
Finally, if you’re submitting to agents, they like to see that you know what shelving category your book should end up in. If you call something that reads like an Adult novel a “YA” novel, they might ditch your query simply because it looks like you haven’t done your research. They’re all about the business: they want to know that your work follows the rules and can sell.
Are there exceptions? Sure. For example, The Knife of Never Letting Go by Patrick Ness. The MC is12, but the book is AR-classified as Upper Grade, typically shelved as YA. Ness was successfully published before he wrote it, so he had latitude. Harry Potters were exceptions - everyone read HP from age 11 to whatever he was at the end. Statistically, though, you’re not that likely to be one of the exceptions.
That’s traditional publishing.
If it’s self-pub, you have much more freedom - but at least you still have to know your target audience in order to package and market to them.