What is YA to you?

I'm just jumping into this subreddit today, so, forgive me if this has been discussed to death already, but I have issues getting my head around what YA really is. Ostensibly, it stands for Young Adult, but even from the start that brings up problems as young is a pretty relative terms. I'm 31 which probably makes me seem extremely youthful in the eyes of a seventy year old, but incomprehensibly old in the eyes of a ten year old. What's more, a lot of people who read YA aren't even legal adults at all. And, on the opposite end of the scale, as the idea of the genre gets older and older so too will it's fan base. There are no doubt plenty of people who say they like YA who would now be in their 40s.

I feel like one could suggest that YA is pop entertainment that isn't written to have much thematic depth. But that would feel disingenuous to me. These authors no doubt are taking their craft seriously and are thinking about what they right. The most ready example of that I could point to would be Philip Pullman, whoose books one could easily find on a YA shelf and have very obvious themes about organized religion, the nature of knowledge and morality.

My next guess would be that YA are books that have more digestible prose. While not necessarily read primarily by 20 year olds or teenagers, they are books that a native reader of a young, but not child, age could reasonably read without trouble. But what is and isn't easily digestible prose is going to be impossible to quantify and I'm sure there are many literary/romance/mystery etc authors who have a very basic reading style that no one would consider YA. I can't think of any modern examples off the top of my head, but right now I'm reading Jules Verne's Journey to the Centre of the Earth and it has very digestible prose (at least in my translation), especially compared to other mid nineteenth century novels. I've heard Verne described as among the earliest science fiction writers but I've never heard him described as a YA author. Though, maybe he would be if he were published today. Maybe Sherlock Holmes and War of the Worlds would be YA if they were published today. It feels like the more I think about it the less clear of a term it is. It doesn't help that basically any story I can think of that would be called YA I could easily assign another genre to it.