How Much Adult in Young Adult?

I’m thinking about my next novel, and the protagonist is likely going to be on the cusp of adulthood, likely 17 going on 18 but possibly as young as 15 going on 16. In most ways, it’s a coming of age story as he transitions from youth into adulthood, but I’m thinking about his age in terms of what kind of book it is. That is, is a book about a young adult necessarily young adult (YA) fiction, and if it is, what does that mean about what themes are off-limits?

I haven’t read much YA fiction lately, but I hear it’s gotten fairly dark. At the very least, I hear it’s not the whitewashed Leave it to Beaver days of Narnia. Probably the most modern YA I’ve read is the Harry Potter series, and from what I’ve heard about other YA series, even the final two or three Potter books were tame by comparison. There was no YA sex, no YA drug use, and apart from some heart-breaking bits in the climax, precious little YA violence. But I hear that a lot of YA fiction these days is all about getting high, getting laid, and getting killed or at the very least beaten.

So I have a very poor feeling on just what the current trend is. I have some YA in my in-pile, and I’m getting to it, but it’s going to take a good dozen books before I have a decent feel for it first hand. I know very well the importance of reading the market, but to some extent I actually have done that. I think of this primarily as a space opera, and I’ve read those for decades. It’s just that my protagonist is going to be in that age range and will be dealing with the bumps and bruises that come at that age as well as a heaping of adult-sized problems. So I’m not sure if it’s space opera or young adult or both.

What makes me question its YA credentials even more is that if it works out, I think I’d like to turn it into a trilogy or a series, and he won’t be 17 forever. I’ve always heard that YA protagonists should be about one to two years older than the target reader. This type of story would age him six months to a year per book, so by book 3 or 4, he’ll be well out of his teens. What’s the upper limit on YA age protagonists? Or does it even work that way?

Anyway, the story is rapidly shaping up in my head as fairly dark, and I’m likely to write it the way I see it regardless of whether or not it’s properly YA or not. I’m just trying to figure out what niche it might fit when it’s done.

Any YA readers or writers out there want to share their experience?