I think it’s hard to sell these type of stories as they want YA to be younger and they want Adult to be older. The manuscript I’m querying now had the character as 19. I just recently made him 18 based on some advice…but I’m not sure it would make sense to go any lower. I’m just hoping that 18 will work okay.
In general, I think YA is 15 to 18 for a lot of agents…or depending on how mature your character/story is.