Fascinating stuff, really.
If you read the fiction in SEVENTEEN magazine (which is pretty much read mostly by 12-15-year-olds, and rarely by girls over seventeen) you’ll find most of it is about 18-21-year-olds. Adults are very willing to read about younger ages (as the Harry Potter phenomenon attests to, since it appeals to adults as much as to children), but teens, well, not so much. Teens are trying so hard to be older, to grow up, to find their own place in the world, one that is not dictated by their parents. And a lot of YA literature is quite edgy when dealing with some pretty harsh topics.
If word count is the only concern, I would just make certain the length fits the story. It’s the story that counts most, that it is told in the best possible way.