Sandra asked: "What makes a book YA vs a book with younger MCs?"
Excellent question! Now, of course, this is just my opinion, but I think an author must consider her audience. I've read many young adult books with lesbian themes and have more often than not been disappointed with the repercussions associated with the coming out process: thrown out of the house, thrown out of school, losing leadership positions at school, harassment, etc. Of course, ALL of these things have definitely happened to some of us, but I wanted to see some happier stories where the main drama is the angst associated with the initial "Holy sh*t, I might be gay!" and then on top of that life-altering discovery is all the normal teenage angst associated with being a teenager (you've all been there so I don't need to elaborate). And, I also feel that "the love scene" in a young adult novel is pretty much the first kiss. One of my beta readers told me that young love is so innocent and I, for one, would kind of like to keep it that way in my young adult stories.
Now as far as addressing your question about the difference between "yound adult" versus "younger MC's" I think you have to consider who the primary reader will be. Are you writing for adults or are you writing for kids? Answer that question and I think you've got it.
There. My two cents (okay maybe four cents) have been given.

Did this help or make it cloudier?
