You always need to ask these questions. Perhaps senior people who have immune systems as 20-year olds, are much more likely to cycle in old age. Or there is some other reason that causes both.
Purely anecdotal: My grandfather is nearing 90 and still jumps on his indoor bike every day. Last winter he fell (due to ice) and got away with a few bruises. For other people his age this kind of fall could have meant a broken hip. He had two heart attacks when he was around 60 and a life long lung disease - far away from the immune system (or health) of a 20-year old.
My point was that living a long and fairly healthy life is a day to day project, not a pill you take once you hit 70.