(This post is part of my "What Age are you Writing For?" series. You can find the introduction and list of age ranges here.)
In general, the older the reader, the more broad the age range will be. While no book is truly for everyone eighteen and older, this is the classification that publishers and bookstores use for almost all books aimed at an adult audience. Whether your book is a self help book for 25-35 year olds or a travel guide for retirees, an 18+ audience all falls under the category of adult or general.
(If you are writing books about early adulthood, college, or first starting a career, you might want to check out the New Adult classification.)
As the broadest age range, adult novels also have the broadest rules. In order to meet reader expectations in this age range, you’ll want to do your research on the specific genre of your book. The word counts of adult novels can vary widely by genre. 60K might be expected in a romance novel, but 150K in an epic fantasy novel. Additionally a sweet and clean romance and a murder mystery will have very content expectations as far as violence and swearing.
But there are some common features of adult novels. One is the age of the characters. Most of the characters in adult novels should be adults, but you can get away with POV characters of other ages in this genre more than any of the others. Children, teenagers, and adults can all be relevant to the plot and behave scenes and chapters from their point of view. However, the main character should be an adult by the definition of their own culture. They should be old enough to have a job, get married, have kids, join the military, etc. even if they do none of these things in the story.
Here are some recommended word counts for some common genres:
General Romance: 70,000–100,000 words (Subgenre Romance: 40,000–100,000 words)
Science Fiction/ Fantasy: 90,000–120,000 words
Historical Fiction: 80,000–100,000
Thrillers / Horror / Mysteries / Crime: 70,000–90,000 words
These word counts change over time and specific publishers have different guidelines, so make sure to do your research before submitting to agents or publishers.