Over the past several days I've received a number of queries that start out like this: "My book, The Greatest Thing Since Sliced Bread, is a series for young readers."
And that ends up being just about all the information I get.
A "series for young readers" is not at all helpful. In your query letter, you should be as specific as possible about who the intended audience is, the age range and the genre of your book. Because if you don't know, the rest of the world sure as hell won't.