I rate books in two different ways in order to make an overall rating. First by how much I enjoyed it (the scale is a simple 1-5 scale) and second by the content the book contains. This is how I rate books according to content:
5/5 - Perfect for any young adult (ages 12+)
4/5 - May have a few bad words, sexual references, or mild violence/graphic family situations or teen issues. Usually written in a informative way otherwise good for young adults.
3/5 - May contain mild to moderate language, moderate sensuality, references to teen drinking, and/or some violence. Recommend for maturer, older teens (ages 14+)
2/5 - Language, sensuality/sexual situation, violence, drug references/use, and/or teen drinking. Shouldn't be read by anyone younger than age 16+. Probably wouldn't recommend it though.
1/5 - Language, several sexual situations, violence, drug use, teen drinking, etc. Not appropriate for teens nor recommended.
*Content will vary in every book, but these are the main bases of my content rating scale.