If your main character is under about sixteen, unless your book is filled with adult themes it'll probably be marketed as YA anyway. That's why Diana Wynne Jones's book Hexwood--emphatically not a children's book (in my opinion, anyway), but with a child protagonist for the main part of the plot--is in YA.
Part of it, I think, is how the author views the underage main character. If you view him/her as a child, even a child with difficulties ordinarily faced by adults, you're probably writing a YA book. If you view him/her as an adult, even an immature adult who has much to learn, you're probably writing a novel for adults. I don't know how to explain it better than this, but I've read an awful lot of YA as well as novels written for adults with younger protagonists, and I can always tell the difference in tone no matter where the book is actually shelved in the store.