Debbie, I don't think 13 is becoming more acceptable--it's been accepted for a long time.
14 and 15 year old MCs, not so much. That's getting firmly into YA territory--the young end, but YA.
Ultimately, when it comes to stories that fall on the borders between two wide categories like MG and YA, I'd suggest that writers write the story to be true to their main character, and leave decisions about category to the publisher, who will have an understanding of what their audience is. I say this as a general rule of thumb--the OP's manuscript most likely is MG, and can be described that way in a query. Manuscripts on the border, though, can be queried as such.