When you walk into the fiction section of a book store, how are the books separated initially? The answer is usually--by age group. There are three major age groups for marketing (only two for publisher's terms, as the first two are combined).
The first age group is children's. For book store marketing, children's refers to anything under young adult. Publishers consider young adult books to also be children's. Children's books are further divided into picture books, early readers, chapter books, and middle-grade. Most fiction books sold are children's books.
Young adult is the second age group. Books in this group are geared toward teens and may contain content that is unsuitable for pre-teens, although that is somewhat rare. Young adult books usually sell less than children's, but more than adults'.
The final age group is adults'. Mature themes and high complexity abound in the category, yet they aren't necessary elements. Thrillers and romance novels are the most popular adult books.
Which group are you most keen to browse?