I’ve written several novels. When I was pitching my first series, a SciFi thriller about immorality, I can’t tell you how many agents said, “if only you’d make the lead characters teens, I could sell this.” Two points: nothing else had to change and it had violence, sex and deep issues about death to discuss. Second, the main character was two hundred years old, which was the point of the story.
Yes, the only thing agents are looking for it MG and YA and now New adult, (20–23??? I think college but pre-parenting). It gets old to be told to write for ya when that’s not your story, but keep the plot with all the adult themes, just make them 17.
I did sell my second series as adult historical fiction, thank goodness, but I could have done it sooner if I made the king younger. But then, it wouldn’t have been historically accurate, so I was able to keep him older. Every other genre has been hijacked by the desire to be young.