Now, I might be tempted to say something silly like, "everyone will like my book," but that's just not true. And, if you tell an agent or a publisher that your book is for everybody, you will likely get a rejection. Why? Because publishers and agents need to be able to market your work, and in marketing, a shotgun approach is rarely as successful as a more targeted approach. You need to be able to tell them who might like your book, so that they can target that audience for sales. This usually means age group and genre. This is also where coming up with comp titles is useful.
This is still somewhat true with short stories as well. When you're submitting a short story you want to think about the subscribers of the magazine or the readers of the anthology your short story might be in. Is it targeted at adults, teens, or kids? Is it a group that prefers quirky stories or serious stories? Might the readers want a more literary piece or genre piece?
Now, when a lot of writers think about writing for a particular audience, one thought that might come to mind is something along the lines of "dumbing down" a story for kids. It's not that children aren't able to handle adult themes, but maybe just reigning in on some of the vocabulary might make a children's book more audience specific.
Hope that all makes sense. Good luck on finding your audience! :)
To find out tips on how to identify your audience, here's a link to an post on The Creative Penn entitled Writers: 5 Tips on How to Identify Your Target Audience
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