Here’s a quick break down of audience categories.
Concept books are geared toward young children, infant to five-years-old. These books deal with everyday life in a very simplified, pin-pointed way. They usually consist of a few pages with very basic illustrations. Material covered is typically nonfiction such as shapes, colors, objects and the alphabet.
Picture books have simple plots and engaging text. Since they are meant to be read aloud, and can be requested a million times, they have to intrigue children and adults alike. Children should enjoy the story and adults shouldn’t be bored to death. The general age range is 4 to 8. The average book length is 20-30 pages and in the ballpark of 1000 words.
Early readers and Chapter books
These books are meant to be read independently by middle grade readers. This age range spans from 8 to 12-year-olds. Usually publishers will designate a reading level for the book depending on its difficultly. The book length can range from 48 to 100 pages, again depending on the level of difficulty. The language and concepts should be simple and concrete. Or at least clear enough for young readers to figure out on their own.
YA books are meant for anyone 12 and older, but for the sake of things let’s say 12 to 18. The topics are more complex, and the length varies greatly. Think about the difference between Of Mice and Men (not sure if it’s classified as YA, but I read it in high school–great book by the way) and the Harry Potter series. The topics are endless but typically deal with a character struggling with self identity, social issues, saving the world issues, issues etc.
These books engage anyone age 18 and over. The length, topic and language very greatly. In my opinion adult books and YA books have such a fine line between them that they’re almost interchangeable. But that’s my opinion and my level of appropriateness.