When you're writing, how often do you think about the reader? If you don't know who you're writing to, you're not writing at your best.
Playing to the Audience
Have you ever been on a stage? If you perform for a crowd, you'll be told to play to the very back of the room. This way, the entire audience will feel included on your performance. Writing is sort of like that, too.
A writer has to be aware of their reader at all times. Think of it like this. If you're writing a greeting card to your 60-year-old grandmother, you're going to use more formal language with less slang (one hopes). If you write a birthday card to your best friend, you might tease them or drop a joke or use some phrase the two of you like. The point is, you probably wouldn't give the same card to both people. And when you write a book, you have to write to your audience there, too.
Age group is the first thing you want to think about. You know if you're writing to adults, teens or children. But which ages, within these big groups, are most likely to be drawn to your story? Think about them. Follow people in that group on Twitter. Write to them. Use references and words that make sense to them.
Don't be afraid that you'll be alienating one audience for another. Write truthfully to your specific audience, and you'll be surprised by how big that audience becomes.