For improving VO2 max, it's recommended that our heart rate is above 90% of its maximum during our hard workouts. If we run too slow on our fast days or run too fast on our easy days, we could plateau early.
If a person has a different maximum heart rate, the 90% would be a different BPM value.
Does that mean the one with a higher MHR has a higher trainability and the one with a lower MHR has a lower trainability?
For the 70 year old athlete, his VO2 max zone might be between 135 and 150 BPM.
Notice from the chart that it might feel moderate for a 20 year old. You probably wouldn't think that 4 x 4 minute intervals at 135 BPM would be very effective for fit 20 year olds.
MHR is known to decrease with age because of the changes in our pacemaker cells. It sounds like older hearts can't work as hard to provide the same stimulus for their muscle cells to get stronger.
We might wonder whether Ed Whitlock would be as fast if he took up running again in his sixties instead of his forties.