Do you have to hire an editor to see if your novel is appropriate for an age group? Because people nowadays self publish a lot, I was wondering how you would verify that your novel is appropriate for a particular age group. There's no such thing as a ESRB Ratings or an organization for rating novels that are self-published, so I was wondering if I need to hire an editor (perhaps not everyone can do that), or if I can hire someone who's cheaper and specialized for this kind of work.
I would personally not get an editor to say what age range your book is for. For one, editors will probably not do anything for free. For another thing, it's fairly easy to tell.
Think of your book as a movie. If the content in your book was a movie, how would you rate it:
G: Nothing too scary or complex. Your novel is simple and not dark at all. Age range: 5-8.
PG: This is the range where you can start to have 'drama'--something huge/not so huge is at stake, and the characters are in danger. These have a wide range of plot complexity but is usually confined to the simpler side of things. Age range: 8-12
PG-13: This would really correspond to a YA (young adult) novel. You can have more frightening and dark things, but nothing inappropriate. The plot can be intricate as you want--within reason. Age range: 12-18
R: This is the range with adult content/dark horror/extreme complexity. Basically anything a teenager can't and/or shouldn't handle.
Of course, there is no hard-and-fast rule about the age range. A person who's smart and mature for their age can probably handle something in another category--within reason.
There doesn't seem to be a common, let alone required, content rating system for books. So why would you want to rate your book, anyway? To target the right demographic.
If you search around a bit, you can find places where you can get your book rated, either by editors (for a modest fee) or by a community (free).
Another option would be to just use your own best judgement. For example, you could rate your book on each of the ESRB's content descriptors on Wikipedia, and then aggregate that to an overall score.
One thing to keep in mind, is that there are vast differences between countries. So content that would be considered inappropriate for children in the US or Saudi Arabia might be perfectly acceptable in France or the Netherlands.
One option could be to show on the cover which sorts of possibly objectionable content there are in the book. For example, Dutch TV shows and movies use icons to inform viewers if there's violence, scary content, sex, swearing, drugs or discrimination (as well as a general age rating).