I thought I'd covered target audience somewhere, but I'm blowed if I can find it now, so probably not. I'll just agree with you instead, as it's definitely something a writer has to make some kind of decision on before they begin writing <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />.
The age, gender, and general outlook of the reader will always play a part in whether they enjoy a story or not, and because of that any story has a 'natural' audience, whatever the intentions of the writer may be.
There are crossover hits (Such as the ubiquitous Harry Potter) but in general an author is well advised to decide on the kind of person they are writing for rather than just typing away and leaving his/her readers to chance.
I'd say the age classifications are the main ones:
Children: No sex or serious violence, no romance, no seriously complicated plotting. Children do not see the point of adult relationships and many are scared or upset by extreme violence. There is also the 'parent factor' - in that most parents object to such things in children's books, whatever their child may think. Swearing is also out for this reason. Stories with children, young teenagers, or animals as leads and emphasising true friendship will do well.
Any Age: Mild swearing, mild violence plus romance are all allowed. Graphic sex and violence are still out. This roughly equates to 'Parental Guidance' in terms of British cinema classifications (One up from 'kid' for everyone else <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" />)
Young Adult: Read 'teenagers'. This is all the stuff that teenagers relate to, like first love, friendship, teenage angst and trying to find out where you fit into a world that doesn't seem to care about you. This is a very narrow genre, and very unpopular amongst non-teenage readers for obvious reasons. Such stories will always involve teenage leads, and usually centre around schools or colleges.
Adult: Anything goes, basically. Too extreme violence or really graphic sex will tend to lose a lot of the mainstream audience though.
Within each age classification there are many sub-genres of course. Far too many to even attempt to list, in fact.
As a general rule though, male readers like to know what the hero(ine) is thinking and appreciate violence; female readers like to know what the hero(ine) is feeling and appreciate romance and intrigue. This is not always true of individuals, of course, but is useful as a general rule of thumb. Both sexes like a well thought out and well plotted story.