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 ($100-150 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 ($0 to $50 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.