My Rating System
Each situation may be a little different, but as a whole, this is how my rating system works. I use the movie ratings because people are already familiar with them. My ratings are a little different, though.
G: Clean! No profanity, violence, "physical intimacy," or any other questionable material. These books are great for everyone to read.
PG: Mostly clean. There may be a few bad words or some violence, and maybe a kiss or two. These books are generally good for mid to upper-elementary school and up. (Once again, it also depends on the topic of the book, so you'll need to look at each book rating individually.)
PG-13: These books generally contain more language or violence, or a "physical intimacy" scene. They may also have a heavier topic. These books are generally not good for elementary age children and are sometimes not good for early young adult readers.
R: I do not watch "R" rated movies, so I don't think this rating follows the movie rating guidelines. These books are heavier topics, they may contain violence, profanity, and "physical intimacy" scenes. These books are not appropriate for younger readers. I usually recommend these books as high school or older. Some I think I have suggested college and up and one I remember (the last book in the "Twilight" series) I said you should be married because of the adult content.
These are the general guidelines for my rating system. Please remember that each book is different and may differ from these slightly. If you think I have rated something incorrectly please feel free to leave a comment. Also, I use "intimacy" in my reviews instead of the real word (I'm not afraid to say it I promise....) because I do not want my blog to be noted as an explicit blog.