Well -- I'm 68, and can still put in a pretty good day's work. I try to be sensible, and break up the workload -- mostly by spacing digging/bending/kneeling work inbetween mowing (for instance) and/or barnwork (which is standing work). I also make a point of taking a break every now and then -- especially in the worst of the hot/humid weather. And I have been known to take the occasional nap (which makes me feel guilty). But I can't go from early morning til after dark, like I used to do. And I sure hate that because I used to get an awesome amount of work done.
Oh well -- I try to be grateful for the good health and strength that I still have -- and hope it lasts for at least another ten years. It'll take me that long to get my Iris beds in shape!!!!