Interestingly enough, I encountered the same problem several years ago flying CX from HKG-JFK, I am a US passport holder and I was 15 or 16 at the time. However, the kicker was that I was flying with my younger brother who was 12 or 13 at the time. They had to call over a CX rep when we went through immigration who explained that at 13 they don't have to be a UM and they pay full fare and thus can bring a child along with them. The immigration officer then asked where we were going and how we were getting there; he winced slightly as I said we had a car to pick us up. Good intentions, however, slightly annoying since both my brother and myself have been frequent flyers since birth.