Writing for an Audience: Intention or Coincidence
When writing any piece of literature, especially a novel, you have to consider who is going to read it. Most of the time you may have a general idea of the target audience you want to reach with your work - children, young adults, etc. At other times, you may be unsure of just who your work is going to appeal to. Before you go branding yourself as an author of stories for a certain age group, you must first consider who you are as the writer.
Before you choose an audience, you must first recognize certain traits within yourself. As you write, you are also the first person who reads what you have written. If you like it, what does that say about you or the people who also would like it? If there is a distinct difference between what you write and the interests of the people you are trying to reach, you may have to make changes to one or the other. However, before you decide that drastic changes need to be made on your end, remember to evaluate yourself fairly. Certain styles or tones that you feel comfortable using may not appeal to the people in your target audience, but that does not necessarily mean that you're the one who has to change. Once you are comfortable inside your own skin and writing in your own way, let the chips fall where they may in attracting readers of certain calibers.
If you're just starting out, pass your drafts around within your social circle to see if your story catches on and with whom. Online forums these days are probably going to be the venue for that, but there is also something to be said for personally handing out physical copies of your rough drafts to real people who can give you honest feedback. Joining a writing seminar at your local library may be useful as well. The instructor and other members of the group can help you with this and other issues you may be having with your writing. Professional writers usually leave branding up to their agents or publisher (though having some idea of your target audience may be useful in choosing a publisher in the first place). The marketing department of publishing companies usually know what they are doing in terms of helping you reach your target audience or matching your material with an appropriate audience.
Keep in mind that books can reach across many divides like age and sub-culture. The Harry Potter books may be filed under children's literature, but people of all ages read them. The "jock" and the "nerd" may also find they share common interests in books that they read. You can try and aim at one or another, but the truth of the matter is that your "imagined" audience is never going to exactly match up to your real audience in the end.