At the 2009 Rutgers's conference, our five-on-five panel leader asked us to go around the table and tell everyone two things. (1) our name (2) the age we felt inside.
I am clearly a four year old. A very smart four year old, but a four-year-old nonetheless. Some of us were 8-9. Others were 11-12. But for most of us it was easy to answer. And THAT is the age you should be writing for.
I have seen lots of people who write picture books who should really be writing chapter books. At the first page session I attended Sunday, at least two people (out of the half a dozen who attempted PBs)) were told, "This sounds like the first page of a chapter book."
If you've heard this comment about your work more than once, chances are you are writing for the wrong age!