Young Adult (YA) is a genre generally written for and enjoyed by readers between 12 and 18 years of age. You can also find broader guidelines, such as 16 to 25 years of age…pushing us toward New Adult (NA).
New Adult (NA) is a developing genre generally written for and enjoyed by readers between 18 and 30 years of age. It deals with issues that only “new” adults would deal with: first jobs, college, getting engaged, marriage, having children, living away from home, etc.
Since there is a cross-over between the two genres, a book can often be described as fitting within both.
Some YA books will be more suitable for mature readers and some NA books will be lighter in theme than some YA novels.
Both genres can and often deal with serious topics such as dysfunctional family dynamics, depression, sexuality, coarse language, and violence.