YA or MG Confusion

Until now, I thought that my two fantasy stories with characters in their early teens would be considered YA. Now, after reading Twilight, I see that YA is more for older teens and perhaps with main characters in their early twenties. Does age matter or is it a style of writing? Difficulty in laguage style and vocabulary? What distinguishes an MG from YA? Since I've been pitching my first novel as YA and though the writing may be considered YA, the characters are young. Though I've read so many books in both, I'm confused and ashamed that I'm still unclear of the difference...