Kitten Adult, I guess. There is nothing in this book that I would consider "disturbing" for younger readers, but I feel like the writing is rather suited for adults (not very colourful, very matter-of-fact). Moreover, young readers would probably not be able to identify with any of the characters (except for Naomi, maybe, but she hardly ever appears).
April I would say, based on the types of crimes that the Alex Cross series is about, generally 18+ would be best. However, I guess there are some teens who would enjoy these. Definitely would depend on the teen, though, and I know that I, personally, would not recommend any book that focused on sexual predators/sexually violent crime for anyone under 18.
Julia Hörmayer Why does everybody say it's for adults only? I started reading his books when I was like 12 and loved them, so no. Teens can read it, too.