Heart rate is affected by so many things. What was your training like the week and weeks before the race? You might have had high training stress on your body, this would give you a low hr. Yes hr changes with age, fitness level, emotional or work stress etc etc. The best way to determine your Functional HR (that is not your max) but about 95% to even 105% of your max, would be to take the average of your last race or two. How long ago was your previous race?