I don't see a lot of 70+'rs out there on the trails, but I do see a lot of them in the obits. I figure that ANY exercise - bicycling, swimming, walking, resistance training, etc., is far better than almost all the folks I know in my age group, who, if they do anything, it is so little, it is of minor help.
So, I ride, swim, walk, lift weights, etc., for the fun and enjoyment and figure that there are also some great things happening to my body. If not, so what? But, I suspect that there are. However, to program those out in some sort of regimen that I am required (even if only in my mind) to follow would simply ruin the whole thing for me. Besides, I love to hear the birds sing and see the deer along the creek beds, catch a glimpse of a coyote or a beaver, and watch the sunrise, and see the thunder clouds, and say "Hi!" to fellow riders, and sometimes even stop and chat for a bit.. YMMV and that is OK, also.