Anyway, I posted a question on the Daphne Unfeasible blog a while back, and the agent, Kate Testerman, responded! Here's some of her answer:
Now, there is a lot of crossover potential between these age ranges, and as we all know, we expect kids to read up, so a 12 year old may expect or want to read about a 14 year old.... Let an agent decide what your book is, and how she can market it to an editor. Yes, she may suggest a change in your protag's age -- so be ready to defend or consider if your story would still work if your MC was 14 -- or 16.Which doesn't give me an easy answer for my dilemma, but does make me think that it might be OK for me to just let my book stand as is. I'm not entirely sure about her suggestion that I just leave off the age range in my query, but I am starting to be a little less obsessed with the question.