Make sure your narrator and his/her narrative voice jibe. Although I’m currently reading Catching Fire by Suzanne Collins, this week I picked up another YA book my 14-year-old son had discarded on the dining room table. I read the first three short chapters to pass the time while some photos I had taken were uploading, and when I was done, I took a peek at the back cover and discovered that the narrator of the story, whom I had surmised to be a 12- or 13-year-old girl, was actually intended by the author to be a 17-year-old young woman.
This took me by surprise. Being the mother of a 12-year-old girl, I felt that the narrator was familiar to me and had a personality similar to that of my daughter — the wide-eyed innocence of a girl on the brink of adolescence. To me, a 17-year-old young woman, who was supposed to be from New York City, would have acted a bit more maturely. The story had an old-fashioned sensibility, which was fine, but discovering the narrator was actually a 17-year-old made her came across as inauthentic. Even my son dismissed the book after only reading a handful of chapters, calling it “cheesy.”
I wondered why the author decided to make her narrator 17 when I thought the story, as is, would work much better with a younger girl. To be fair, perhaps I’d have a different impression if I had continued reading, but it was a reminder of the importance of having an authentic narrative voice. If you decide to write your book from the perspective of, say, an eight-year-old boy, be sure you know the way an eight-year-old boy not only speaks and acts, but thinks. And consider the time period. Had this book been a work of historical fiction, I think the author may have gotten away with having such a naive 17 year old. But since the setting is modern-day, this narrator is competing with the Katniss Everdeens and Bella Swans of the world for the attention of today’s savvy young adult readers, who know a phony when they see one.

Very good point. If the age of your character does not ring authentic with what he or she is doing, it will sound phony. Yep!