there will be a test

Vocabulary in middle grade books (meant for kids 8-12) is controlled, the books are only between 25-40K, and the sex and death aren't shown as graphically or lovingly as in adult novels.

Got it.

But what about YA?  Apparently, it's aimed at teenagers or older pre-teens, but a good-sized minority of readers are adults.  Judging by the books I've picked up at the library, the themes tend darker and angsty-er than most adult books (of course, maybe that's because I pick up the super dark and angst-ridden titles ;) ).  I've read to aim for about 40-60K, but often hear about people shopping 90K manuscripts, and a lot of the books at the library seem long.   I've read that the major difference between YA and adult is pacing- in YA books, every page has got to be interesting in its own right.  But shouldn't that be true of any book?!

What are the rules for YA?  What's the real audience for YA books, and what kind of frame of reference are they assumed to have?  

Is the vocabulary the same as in adult books? I don't want to chock my story full of references to things like "tulle" and "maypoles" if the audience won't have been exposed to those things before- but I also don't want to talk down to them and assume they haven't heard of the things I have.

My story also has a lot of brain-teasers and puzzles in it, and those need to be hard without being impossible.  I'm around high school students a lot, but I'm not sure how hard is too hard for them.  At that age, there seems to be a HUGE range in how comfortable they are with abstract thinking, because there's a HUGE range in how much algebra, music theory, and art they've been exposed to so far.... I'm worried that will mean a HUGE range in how hard or easy the riddles are for kids, so it'll be impossible to hit the difficulty right.

I don't want to talk down to a YA audience, but I also don't want to make reading my story into a chore.  What's the line?  Or is this even worth worrying about?