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 <PERSON> 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! 11