Yes, many more adults are reading the YA stuff now, because it IS so good, and doesn't spend half the book justifying that it's fantasy. Yanno, added blood, gore and sex just to push it off the youth shelf at the bookstore. Personally I can't get enough Tamora Pierce, Rick Riordan and James Owen, just because they write SUCH GOOD STUFF. They're YA/MG books and I adore them.
MG books tend to run the 30-60K range, YA between 50-75. The caveat is that since Harry Potter, MG and YA fantasy have allowed longer wordcounts to see the market. Right now I'm reading Shannon Hale's The Goose Girl and it's a good 300 pages. It was in the YA section, and it's completely fabulous.
Reader ages:
MG usually 9-12 (Independent reader in the bookstore)
YA usually 13-18(Youth or YA in the bookstore)
There is a 'new' sort of category emerging, the 10-13 age group, where the books are a little above MG but not quite YA. Just write the book, let the publisher figure out where it goes in the bookstore.