Target Group of Readers

For all books, there is a target group of readers. Kids like short, colorful books. Teens like longer, more emotional, adventure books. Adults seem to like nonfiction books more than younger people.
For my novel, I am having a difficult time understanding my target group. I am almost 19, but I have been working on ideas for this book since I was 7, so I think my target group changed as I matured. So I have no idea who would read this now...
The book follows the main character from her preteen years until she is middle-aged. It is a fantasy book, so about half of it is based in a universe I made up; the other half is based on Earth. In the fantasy universe, there are monsters and ghosts, and the characters are divided between "good" and "evil". On Earth, everything is different, where there is a fine (almost invisible) line between the two. Some major characters include serial rapists, prostitutes, and murderers; and not all of these are antagonists. There are all sorts of criminals.
The fantasy aspect seems appropriate for preteens and teens, but some of the more disturbing parts of the story should probably be reserved for more mature readers. However, I don't think adults would find the fantasy world very interesting at all...

Based on this little information, does anyone have any idea who my target group should be? Any ideas on how to better the basic story? I greatly appreciate it...