There were a couple of threads on this. Actually, someone asked this question to Nathan Bransford on the "Ask The Agent" forum.
Here's a
link to the forum.
I think the conclusion we came to on the other forum was send out a couple of letters to agents where you say your age, and if they don't do well, try some where you don't. I guess it depends.
Here's
the other thread with this discussion (or, one of the other threads).
With all due respect to Nathan Bransford, I think you should take his advice with a grain of salt. It may be perfectly true that he would prefer to know, but you have to ask yourself why.
Finding a publishable novel in slushpile is rare. Finding a publishable novel written my a teenager is even rarer. It could simply be a very effective weeding criteria. Good for the agent, perhaps, but is it good for you?
I would not mention it in a query letter. The query letter is not the place to sell the novel. The query letter is for piquing the agent's interest and getting him to ask for a partial (or hopefully the whole manuscript). You don't want to include ANY sort of exclusion criteria. Many agents have interns to screen query letters, and their job is
not to find good books; it's to
exclude as many submissions as possible. Even those who don't use interns are looking for quick ways to reduce the slushpile.
But once you have a request to submit (partial or whole), that might be a good time to mention it in the cover letter. At that point, they won't reject it out of hand because of your age, and it might just become a selling point. The agent also won't feel like you tried to hide something from him as he might if you waited until he asks you to sign with him.
BTW, this advice holds true for job hunting as well. Just substitute HR employee for intern, resume for query letter, and interview for manuscript submission.