This is one of those oft-asked questions in the publishing world and, to be honest, I'm not sure why it's such a difficult one to grasp. As with any genre, YA has exceptions and oddities that are not so easily defined, but the basic definition is almost always the same: YA is fiction marketed at young adults that typically features young adults as the primary characters. Beyond that, there really isn't any clear difference between the two genres, except, perhaps, that YA has a tendency to contain more bizarre fiction series within its walls.
The thing about YA is that it often gets misinterpreted as a genre that must appeal to a particularly young age. People make the assumption that a YA book shouldn't deal with what they perceive to be adults topics such as sex, drugs, etc. But it doesn't take a genius to realize that not only are teenagers and even middle school-aged kids talking about all these "adult" subjects today, they've been talking about them in previous generations too. This subject has been in the teenage sphere for decades, with traceable origins at least to the sixties, and likely even further back. It's not like our teenagers are wholly innocent anymore; they are just as interested and concerned with "adult" subjects as adults are. So, it seems fitting that a genre that is meant to appeal to them would contain topics that they are already discussing and already trying to understand. And if they have to get their knowledge from somewhere, better a book than a parent who is unwilling to discuss these things--I believe that parents have largely got what they deserve in their kids today by not being active enough in their lives to even understand what the teen struggle is like now.
That said, there is plenty of silliness in the genre, as characterized by many of the quirky fantasy series that permeate the shelves. That's okay too. There's no reason why a genre cannot have its silly moments. Obert Skye is to YA what Terry Pratchett is to Fantasy. But at the end of the day, YA isn't about a particular subject so much as a particular marketing element; it is not a juncture between middle grade fiction and adult, nor is it a depository for pointless fictional drivel. It is a serious genre that deserves credibility just as science fiction or fantasy do, and the fact that it is still under appreciated for what it provides society is disturbing at best.
And that's that!
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