Concerns about fatigue

Should I be concerned because I do not seem to have any "get up and go?" Many older people have much more energy than I do. I just run out of gas toward the end of most days. Of course, I do not get much exercise, and that may contribute to it, but it still seems worse than it ought to be. I cannot attribute it to anything medical, because the ailments that I know I have should not affect energy and stamina. Puzzling.