I find it hard to make a clear divide - Deep Secret has never seemed to me to be clearly completely different from The Year of the Griffin (or even The Dark Lord of Derkholm, which is quite a bit about Derk and Mara, and a *lot* about young people dealing with very adult things) - and then you've got Hexwood, which is written like YA but almost all about adults, and...

But I haven't read A Sudden Wild Magic, and it sounds like I should - and even like it might be in a category of its own :)