Fun & Fitness over 50

I just read something which I think is astounding; that maximum heart rate is fixed for life and the commonly used formulas for calculating it are useless! This from the book Heart Zones Cycling by Sally Edwards & Sally Reed which I am currently reading.
I mean I know that a formula is only going to be an approximation and I've already proved the most common one wrong; the one that says your MHR = 220-your age. For me, at age 59, that is 161. I've already observed higher rates while cycling with my heart monitor. But the concept of the formulas is that your MHR declines with age or varies by age and weight. These authors claim that research shows that MHR is fixed and does not decline except in cases of chronic unfitness. The only way to determine MHR accurately is to test it. The authors use the Foster Talk Threshold Test where you find the point at which you are starting to have trouble reading a 30 second test and then you can calculate your MHR and Threshold Heart Rate.