First of all no two humans are the same. So don't put too much faith in the numbers. I am 5 months away from 70 and can reach high high rates of 175 beats per minutes. First thing first. Enjoy your workout without looking at heart rates numbers. Do 20 or 40 minutes depending on how you feel on that day. Every day is different depending how good a sleep you had the night before, how your digestive system performed the day before and so on. The most important part is that you do it!
Peace and Good Health.