Use a graphical model
the maximum heart rate for a healthy 85 year old
As we get older the maximum heart rate that is safe declines. What is the relationship, what equation should be used? If you look on http://en.wikipedia.org/wiki/Heart_rate you will see that there is quite a debate and that various equations have been proposed.
One formula that has been used is: maximum heart rate in beats per minute = 208 - (0.7 x age in years) and if you work out the max. heart rate for ages 0 and 100 then you can plot a graph. Put age on the x axis and max. heart rate on the y axis. Because the number you have to subtract from 208 is directly proportional to age it means that the graph will be a straight line. You could calculate the max. heart rate for age 50 and check that it lies on the line. Now you can use the graph to work out the max. heart rate for any age between 0 and 100 years.