There are two questions to consider in the Primary Audience section. 

Sexually Explicit Images or Title – This field is mandatory. You must select Yes or No to this question.

Reading Age – This field is optional, but it is essential if you are publishing a YA or Children’s book. Selecting appropriate reading ages ensures your book will show up in the correct searches and reach the intended audience. 

A screenshot from the KDP publishing process. The heading is 'Primary Audience'. 

There is a place to indicate if the book contains sexually explicit images or title, and an optional area to indicate the intended reading age for the book.

You can find more information about Primary Audience here: https://kdp.amazon.com/en_US/help/topic/G201506310