Adult protagonists are adults. It seems simple, right? Seriously though, if you’re writing an adult novel or series you’ll want your protagonist to be 25-years-old or older. But, as all age ranges blur, your protagonist could be as young as 20.
Adult books tend to focus on situations adults deal with—divorce, marriage, children, careers, and aging parents, among other topics. Genres are just as expansive as in MG, YA, and NA books. This means these ‘adult’ topics are handled in whatever setting is appropriate to the associated genre.