It all depends on the person. Some of us are more lucky than others, and it makes a difference in how much you exercise. I for instance am 75 and my legs and muscle mass is great for my age. I usually ride somewhere between 2 and 3 thousand miles a year, all during the warm months, since I live in the snow belt.

The big point here is keep on doing as much as you can. If you set you rust. If you keep biking, your decline will be much slower.