A lot of young adult books specifically speak to teenagers through characters, voice, theme, and tone. For instance, many books for teens will position a young person as the protagonist so that they can better relate to the intended reader and share insights that may be relevant to young people in general.
However, there is no definite answer about what makes a particular book appropriate for teenagers. In fact, there has been a longstanding debate over what kind of books are appropriate for which age ranges, the content and themes teenagers are equipped to deal with, and the types of books that should be found in schools.
In many cases, reviewers and publications will provide particular books with age-appropriate ranges. These age ranges act as suggestions rather than hard rules, but they can often serve as an easy-to-reference guide when shopping for good books for teens.