My heart rate monitor is based on my age and weight. It then calulates what my target rate should be and I have a beeper to tell me if i am going too hard or too slow.
It also gives me a fat buring range and a cardo range up to the 90%. I find that at 46 years old, my range is anywhere from 112 to 165, but I know that younger totally fit people around me are shooting for a much higher average.