Hannah Moskowitz has written a really interesting post about the differences between MG (Middle Grade) fiction and YA fiction, the first which she defines as being for ages 11-14 and the latter for ages 15-18. She says that MG fiction is more of a protagonist-against-the-world situation, where the main character is trying to fit in, whereas YA fiction gives us protagonists dealing with personal problems focusing on very specific relationships, and the group is less important. (This is a very brief summary; for a more comprehensive analysis, read her post.)
Personally, I feel the differences she describes relate more to genre than to age, and that both kinds of books can be found for readers ranging from the very young to those who no longer count their birthdays. What do you think?