I have written blogs about genre before (see: What is Women's Fiction? and What is Contemporary Fiction?) but this one is going to look at two major factors that can determine a book's genre: Age Range and Length.

Now, first up, I need to discredit this post before we even start and say that there are not really any hard and fast 'rules' to all of this. There are a bunch of different factors to consider. I am unable to say that if the target audience is 12 years old, then the genre you are looking for is 'Children's Books', or if the book is 40,000 words in length, it is a novella. It doesn't work like that, exactly (and you're going to see why). But, I can give you an idea of industry averages, and give you a place to start looking at what people say.
So, let's do it! Starting with...
GFNRE BY AGE RANGE
What do I mean by 'Age Range'?
This is the ages of the readers that the book is targeted at, or the 'recommended age' of readers.
Youngest to oldest:
Children's Books
The 'Children's Books' genre can be broken into smaller categories:
Baby to 2 years old - Baby books / Board books
Up to approx. 300 words
3 to 5 years old - Toddler books / Picture books
Approx. 250 to 1000 words
Writer's Digest says that the standard text for Picture Books is 32 pages.
6 to 8 years old - Early readers
2,000 to 5,000 words
Younger School age / Chapter Books
8,500 to 12,000 words
9 to 12 years old - School age / Middle School books
12,000 to 50,000 words
12 year olds / Tweens - Upper Middle Grade
40,000 to 55,000 words
You can see that some of these categories cross over in terms of age ranges. Also, different places use different age ranges, category names, and word counts. I have listed all of my references at the end of the article, so you can find out more if you like. None of them differ hugely, but I'm not about to say that these figures that I have given are the be-all and end-all. If you are an author doing your own reserach for a book deal, or something important like that, check with the publishers themselves about what expectations they have.
Okay, now, I didn't know this before now, but 'Children's books' are not aimed at children above 12 years of age.
After that, they move into...
Young Adult Books
Young Adult books are aimed at children between 13 to 18 years of age.
They are roughly 40,000 to 80,000 words.
New Adult Fiction
New Adult fiction is aimed at people who straddle the Young Adult and Adult categories.
They're still kids, growing up, but they are also adults.
Aimed at people between 18 to 20 years old (although Wikipedia tells me that protagonists in New Adult fiction could be 18-29 years old. I'm thinking that if a book is aimed at a 29 year old, it's classified as Adult Fiction. Although yes, a protagonist could be 29 and the book could still be aimed at an 18-year-old. Please give me an example of this!).
New Adult books are 50,000 words and above.
Adult Fiction
No, this doesn't automatically mean that there is 'adult content' (think: sex, drugs and rock n' roll), heck, most young adult and new adult books are going to touch on these themes too. But adult fiction aims at people who are 'real' adults (lol), over 20 years old. They've done a little growing up.
Keep reading for a more specific idea on word length of adult fiction novels. It kinda depends on the book genre and can be anywhere between 50,000 and 110,000 words.
Mature Fiction
Okay, so there isn't really a category for mature readers with protagonists over 60, but there probably should be. Amazon has a category for 'Later in Life Romance' and there are a few authors who do write for an audience over 60 years of age, like Joanna Nell, who writes books like, The Great Escape from Woodlands Nursing Home (which I loved, but thankfully, am not over 60... yet). Some pages like, Book Riot, and Goodreads have compiled lists of novels for women over 60, so do check them out. Do you think 'Mature Fiction' should be a thing? How would you find fiction books for the over 60-year-olds?
Although not a genre, perhaps I should also add in here that there are large print books available for readers who may struggle to read books with small print. So... yeah...
Moving on to...
GENRE BY BOOK LENGTH
Like age range, there are no hard and fast rules. The other thing to consider here, is that Genre often determines the length, not length determines genre. This means that If someone wishes to write a science fiction novel, they may be aiming for 100,000 words, but if someone else wants to write a romance, then they may only need 70,000 words. 'Novels', as a general category, has a wide length range.
Let's look at word length from smallest to largest:
Micro-Fiction
Micro-fiction can be super short! I entered a micro-fiction competition years ago, that was only 50 words. I had to use 50 words exactly, no more, no less.
Micro-fiction can be between 50-500 words.

Flash Fiction
Around 500 to 1000 words.
Short Story
1500 to 30,000 words.
Novelette
Kindlepreneur uses the category 'Novelette'. They suggested that a short story is 1000 to 10,000 words, a novelette is 7,500 to 20,000 words, and a novella is 17,500 to 40,000 words.
(I think that if you have a 30,000 word story, you can take your pick of categories from 'Short Story', 'Novelette', and 'Novella'. I don't think it's going to matter too much.)
Novella
The Write Life says: 30,000 to 50,000 words
Kindlepreneur says: 17,500 to 40,000 words
Whispers to Roars says: 20,000 to 50,000 words
The length of a novella seems to differ a lot depending on where you go to look.
If you're interested, the length of my novella, The Silver Maid, is around 26,000 words.
Novel
Anywhere from 50,000 to 110,000 words
By Genre:
Romance - 70,000 to 90,000 words
Sci Fi - 70,000 to 100,000 words
Historical - 80,000 to 100,000 words
Fantasy - 90,000 to 120,000 words
Thriller - 70,000 to 90,000 words
Be aware that there are genre expectations!
If you want to write a romance novel and try to publish a 110,000 brick, you will probably miss out on a big chunk of audience who will be turned off by the sheer size. You may also find that publishers won't even take a look at your work if it looks too big or too small. Do your research.
Epic
An Epic is anything over 110,000 words.
These books are hefty and generally involve a lot of world-building.
Any questions? Let me know and I will endeavour to find out!
Happy reading,
Marley x
References:
Journey to Kidlit - The Age Levels for Children's Books You Should Know
Miriam Laundry Publishing - Children's Book Age Categories
Amazon - Children's Books: Shop by Age
From Whispers to Roars Blog - Average Novel Word Count by Genre and Age of Readers
The Write Life - How Many Words in a Novel?
Kindlepreneur - How many Words are in a Novel? Word Count by Genre and More
Writer's Edit - 3 Key Differences Between YA Fiction and Adult Fiction
Writer's Digest - Word Count for Novels and Children's Books: The Definitive Post | How Long Should a Book Be?
Other links can be found throughout the blog post to take you to places for more information.