I'm still in my 30s, but I wonder how much mental decline is actually due to physical decline. I notice I feel more sluggish, sleepy, less sharp and motivated during periods when I'm more sedentary. And while exercise is tiring, I feel it gradually improves not only physical stamina, but mental stamina as well. Clearly a large part of our brain power is spent controlling our bodies, for when a stroke happens, becoming paralyzed in an area of your body can result. And clearly our body caters to our brains needs (e.g. nutrition), so if the body declines, then it shouldn't be surprising to see mental capabilities decline as well.