The Flesch Reading Ease of the Soul in Words series of books:

The Flesch Reading Ease of all three books in my Soul in Words series averages between 70 and 80. This means based on that score alone, it is calculated to be readable for someone in the seventh grade or higher, which is typically at the age of thirteen or older. 

This is another dimension to consider when rating my books as PG-13. Thirteen and up is the audience I set for Amazon and Kindle. It is up to parents to decide. Some of the topics in my books are intense at times. Without parental guidance, I would say sixteen or older would be more appropriate.