I've heard other people say that the 'fat burning zone' was invented to sell more stuff, don't know if that is true. I do know that like Meg says the 'fat burning zone' is a myth.
It is basic math - the higher your heart rate the more calories you burn. As you burn more calories the percentage of fat calories burned compared to regular calories burned goes down, but the number of calories and of fat calories still goes up.
I'm 36 and I keep my heart rate between 155-170 regularly, often get up to 185 without really feeling winded depending on the workout. My new HRM ranked my fitness at 42, which it called 'very good', anything 44 or greater was considered 'elite'. I know I built up to that strong a cardiovascular rating from keeping my heart rate higher than the 'fat burning zone' for the last eight months of exercise!
You sound like you are in a great range at 155-160. It will do nothing but good things. Good luck!