There are some very interesting posts on this thread. This is a subject that has interested me for years.
I have run all of my life. After age 30 or so my main interest has been long distance running on the roads and the marathon running. I have observed the slow-down among the 50 runners who have been a part of the running group that I have met with on Sunday mornings since 1971. My track bests were all run in my mid to late 20s but a number of my best road times were set at age 35 and my best marathon time came at age 36. In my case training mileage and physical problems had a great deal to do with this.
I feel that long term marathon training/racing has an affect on the immune system. Additionally, I have a list of over 17 long-time hard, serious runners, many of them friends, who have had arterial fibrulation or flutter and that includes myself.
As seen on this thread, running/racing is a very individual activity and it can be affected by many variables including how hard and how much one trains and races. Also, as Coach Mihaly Igloi often said "Must have big goals". But although they have to be strong, goals have to change as one ages. And then, as one ages, surprises (cancer, Parkinson's, Alzheimers} develop depending on the genes the runner has inherited. There is luck involved.
In spite of all this, the runners who have slowed the least are those who have maintained top fitness year after year without taking prolonged breaks. As we all find out, getting in shape again for the older runner takes longer and longer and recovering from injury gets harder and harder as the years fly by. And, in addition to all this, the one key to longevity in running is to never, never give up. By the way the biggest slow down seems to come between 75 and 80.
I am currently walk/jogging but still hope to eliminate the walking soon. I wish you all every success as you continue to enjoy the sport of running.