: How to figure out what my target readers are? I write something between fantasy and sci-fi. But if someone would ask me what age group, target group I'm writing for I wouldn't know how to

Target audience is all about selling the book, and marketing the book. It doesn't necessarily need to be a concern to you at all when writing it, especially if it affects your artistic decisions. But you do need to figure it out to sell it.

The question is who would be most likely to buy this book, and how do you convince your potential publisher of that fact? Here in the United States, fantasy books (that aren't epic fantasy) tend to be marketed towards children because there's a healthy youth fantasy market. In Britain, there is a strong adult fantasy market, so sometimes the same book is marketed for adults over there, and kids over here. This sometimes leads to situations such as the Golden Compass series, which was written for adults, and arguably inappropriate for children, but which had a child protagonist, and was targeted towards children on this side of the ocean.

So, if you're writing in America, and you're writing a fantasy book, and it doesn't have anything in it inappropriate for a six-year old, your target audience is de facto youth. The actual age will depend on the complexity of your word choice and plot, the age of the protagonist, and the length of the book (longer means older). Your target audience is probably a couple of years older than the grade-level the reading complexity tests at (people don't want to work too hard in their entertainment reading), and a couple of years younger than the protagonist (children tend to identify with a protagonist just a little bit older than them, it's aspirational). If your protagonists are grownups, your target audience is probably teens.

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To me, A writer's first target audience is themselves. When Stephen King was asked why he wrote horror, his reply was, "What makes you think I can write anything else?"
The bulk of my fiction has always been fantastical in nature, sci-fi or horror or fantasy. An exception was my father's mystery books, Rex Stout, John D. MacDonald and some others. I enjoyed them, but after reading wanted to get back to the aliens and talking dragons.
I write stories I would enjoy reading. More broadly for those that enjoy the books I like. If I wrote for children, it would be books like I enjoyed as a child, or that I enjoyed sharing with my child.
I honestly cannot imagine trying to write fiction that I do not enjoy.
Look at your bookshelf. Then look up the titles, and try to find the demographic for it.
Added, partially from my comment below, to provide more details:
It is fairly safe to assume that if you enjoy reading something you are in the target demographic. Of course there are always weird outliers, like tatted up maximum security alpha male inmates enjoying Little Women, but unless you know you are an outlier, presume you are somewhere in the middle of the appeal demographic.
Here is a link to real life demographic outline for Star Wars characters, and although this study focuses on popular entertainment, the measurements are typical of what demographers are studying: Remember, the point is to target marketing and thereby reduce the cost of marketing your book, and to judge the size of the market. Interestingly, bigger is not necessarily better: Very few products appeal to everybody, and in entertainment, even a million-seller has sold to less than 1% of the total readership.

Luke Skywalker fans are mostly male and middle-aged, earn higher salaries compared to other fans (0-150 thousand), are most likely to work in business.
Darth Vader's fans skew older and earn more than fans of other characters, are likely to work in criminal justice. 2nd most popular character among females.
Princess Leia is the most popular character among females; her fans are typically over 45 years old, and tend to work in software.
Han Solo's fans are the youngest of Star Wars fans and have the lowest salaries ([CO] to thousand); and are most likely to work in retail or construction.

And so on, there is a second page but you get the idea: Fans of Finn (the former StormTrooper) tend to work in military and manufacturing.
The industry focuses on age, gender, income and profession because these are some of the keys to targeting advertising in print, TV and radio ads and everything else. A marketer can find average income down to the block, it is even computed for some billboards: The average income of the people in cars passing this billboard every day is $X. We can find military towns, can find software towns. It is easy to find advertising outlets that are more male oriented or more female oriented.
So the first advice is to realize your demographic is to carve out a LOT of people; so while it is specific it is specifically a broad cut of the population, and it is a series of such cuts that chop your demographic down to a manageable market.
Bigger is not necessarily better. Marketers want rich fields to mine, at least to start. They have to pay for every impression (viewing, hearing, reading) of an advertisement, not just the impressions that resulted in sales. So this isn't a question of whether 6 year olds can enjoy your story, but what percentage of 6 year olds will enjoy your story. Or what is the most likely age of a reader that will enjoy your story? (or age range; e.g. 18-44 is a common range).
The marketer's dream is to zero in on a small segment of the population that is ten times more likely than the average reader to buy the product. If somebody writes a first homosexual love novel, bingo: Target identified. Upper middle class gay teens won't be the ONLY audience, but they would be a rich field to mine so sales can be made. (similar demographic as Brokeback Mountain.)
So these are the types of questions to ask yourself, and see if they apply. In particular about yourself, but also about your main characters, because the audience will tend to identify with specific characters.

If you enjoyed a work of fiction, at what ages would you have enjoyed it?
Is your work gender specific? Will women like it more than men, or vice versa?
What is your education level? Does it take special knowledge to enjoy the story? Harry Potter's setting is a SCHOOL with children, effectively we are reading about 6th grade through 12th. The audience for that book is much different than the audience for a novel about detectives chasing a serial killer, or a legal fight over AIDs victims, or a story of racist cops getting their due, or a story about corruption on Wall Street.
What income level do you write to?
Will your story or characters appeal more to one race or culture than another? If you are set in white middle class America, the answer is yes. If your MC is black in white middle class America, still yes but a different demographic (The Blind Side).
Where do you lie on the spectrum of devout religious conservative to militant atheist? Consider the movie Juno; with an out-of-wedlock teen pregnant and considering a legal abortion. Eventually giving up her child for adoption, and in the end continuing an apparent sexual romantic relationship with the teen father of the child. Juno seriously pisses off some religious conservatives as liberal propaganda promoting sexual permissiveness without any serious consequences.
Where do you sit on the spectrum of every-man-for-himself free marketer to we-are-all-in-this-together Socialism? Where does your main character sit? Will readers feel like you are preaching the liberal gospel (I like it), or will they feel you are preaching the free market gospel? (I hate it).
Where do you lie on the political spectrum? Consider tax policy, government services, mandatory military services, police protection and other public services. Even in your fantasy: If your characters live tax free and there are no public services (like sewage or water or road repairs or defense), is that taken for granted as good, or is sewage disposal a frikkin' big problem for those trying to live their lives?
Who would your story turn off? Who would your heroes appeal to, or alienate?

We can comprehend that in Star Wars the character of Hans Solo is indeed a lower middle class hero with a lot of bravado and a crappy truck held together with twists of wire and zip ties, with an inarticulate but brave sidekick, a bit irritated by complex arguments but street smart and not stupid. More of a cut to the chase, punch-first and talk later kind of guy. And the audience he appeals to wants to BE him. They probably do not really punch first and talk later, but wouldn't that be great?
The widest possible demographic, everybody, is also a low concentration of readers. The point of identifying your demographic is to narrow this as much as reasonably possible, so the marketer can see a rich field to mine for sales. It is okay to piss off one group and appeal to the other, if we can market to the appeal group and ignore the pissed off group, we saved some money.
So the parameters of your demographic division should ALSO be things a marketer could most likely use to target advertising, things generally known about people. A demographic of "has read Dante's Inferno" or "has visited the Vatican" is kind of useless by itself, I don't have those statistics by zip-code or neighborhood, and would have to generalize them to some level of education and income level.

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The target audience depends on the kind of material you want to talk about. If you want to add in rape, sex, drinking, and detailed violence, you probably won't be writing a novel for 6 year olds.

I used to read a series called RedWall when I was a child, basically an personification of animals that took on a lord of the rings style of adventures. It was written without questionable details and there was no romance to it (to my memory).

So really in the end, there isn't much we can do to assist in this question. This largely comes down to the material you want to talk about and how those topics are portrayed.

Once you assess the material and the topics you want to discuss, you can then figure out your target audience or modify the material to be more appeasing to certain demographics.

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First of all, if you are writing a story and you still don't have a target audience, that could only mean trouble along the path for you.

When writing a story, from the very beggining you need a target audience, so if a time comes (and it will, I asure you), in wich you have no more ideas in how to make more progress in the story, you need to know what kind of shenanigans you can use.

Can I kill the problem? Can I seduce the problem? Can I jump and jest the problem in my favor?, Can I use a song to solve the problem?

Adding to that the fact that if for some reason, you can only think of dark ways to keep going in a story for children, or just funny and silly ways to solve the climax in a dramatic scene, then you will never see that what you need to do, is to change a lot more than a simple scene, maybe even the complete chapter or more.

Now solving the question, how do you choose the target audience for a story?... Quite simple really.

You have to think in what kind of message you whant to say with your story. Is it deep? is it to improve as a human? is to prevent people to fall into a dark pit? is to make heart beat? to make a face blush? to free a mind?

And then when you know what kind of message you whant to tell in your story, you need to ask, which kind of people will benefit more of this story?.

Will a elementary student take something out of a dramatic novel of the slave trade of 1545? Will a politician or a doctor take something out of Winnie Poo´s adventure to magic dreamland?, The answer most likely be no...absolutely no.

And then you can even reduce more the possibilities after you check what kind of public you can reach.

Will this be published in a important magazine for professionals?, will this be pasted in a corner of the church wall? Will this be published in a web-page for fanfics or one for original stories of young authors online?

Believe me, nothing is more frustrating than spending a lot of time in a writing that you work so hard, just to see that 12 or less people actually read it and then quickly dismissed it.

And lastly, (and this is completly optional), use the correct accent or idiom acordenly to the target audience.

I have found that using slang terms is better if you whant to write for young people, they feel more connected with the story and it´s characters.

As for accents, is not bad to use word more commonly use in other contries to denote the origins of said speaker, more so if this work is going to be published to a diferent contry and/or view in the red were literaly, there are no borders.

For example, I´m not a native English speaker, (I'm from México I speak spanish), but I know that more people will benefit from my opinion if I write it in english.

My target is not exactly an audience, (Exept if yoy count all the people in Stack Exchange from the diferent contries of the world), but rater my objective is to help the magority of the people in this site.

So to conlude, the 2° or 3° thing you MUST do is select a target audience, for this propuse I recomend

1) Find what kind of message you whant to tell in your story.

2) Find what kind of people will benefit most of this type of message.

3) See what tools you have to publish your story and ajust the story so you can finish it in time.

4) (Optional) If you think it will be better, use the correct way of expresion, may this be some slang terms or more practical and generical expressions or even other idioms and accents.

Cheers mate!!, and good luck!

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