I wondered if age was a factor in stopping most people running or whether the fact that they run keeps them fitter for longer than their non-running piers?
Both of the above are correct; ultimately, age is a factor, but it is much older than most people imagine.
A good mate of mine as a
Vet 70, ran 69.10 for 10 miles and 3.26 for a marathon. He is now 83 and would find it hard to get under 35 minutes for 5K, though he is still a pretty quick cyclist.
So your other comment is also correct, the fact that he runs (& cycles) has certainly kept him fitter for longer than most people 20 years younger. It isn't just the fitness part, but the mental aspects that are also important - he socialises with other runners and cyclists - many of whom are less than half his age, but that's the great thing about running (and cycling) - he is considered to be 'one of the boys'.