Thoughts About The Level of Reading in My Book

Been doing some research, and have decided that the book I'm writing falls into the Young Adult category. While parts of it will appeal to 9-10 year olds, the majority will appeal to those 12 and older. Some of my critique partners think I talk above my readers, but I don`t believe in "dumbing down" a book. If kids don`t understand a particular word, they will ask an adult, or look it up in the dictionary. The goal of my books is to stretch the reader, both in understanding and imagination. Isn't that what books are all about?