Peter T--I started piano lessons when I was . . . maybe seven? I can't remember now. I guess I didn't think it was the most fun in the world simply because it required me to sit still for long enough to practice (I had this same problem when I took violin for a year at age 4). I don't think it matters what instrument it is--if you're going to practice and learn the basics, it's going to be a little tedious. Incidentally, I started playing the guitar at age 16 and was very glad for the theory I had learned on the piano. Not that you have to take piano to learn it, but I think it was easier to learn it on the piano where you get one note per key instead of trying to figure it out on several notes per string, as one has to do on a guitar.

I've heard that children should be started on musical instruments as young as they can handle it because it "wires" their brains, making it easier for them to learn more later (sort of like learning a foreign language--it's easier when you're little).