Target Audience

If the film is in the horror or thriller genre generally the rating will be 18+ due to the adult themes in the film such as sexual content, violence and offensive language. Filmmakers know that the genre is intended for people over the age of 18 and while they are writing the film and directing it they don't need to change their film to suit different ages. This is why often horror/thriller films are contreversial due to their extreme violence or disturbing ideas.