If accepting the fact that you will one day die is accepting reality and the nature of life, then so is accepting the fact that increasing age is accompanied by weakening physical powers and abilities. Yes, you may rightly forestall decreasing abilities with proper exercise and diet, but you should accept that it is just a forestalling of what is inevitable. You will not serve yourself or the quality of your life by bemoaning reality, by thinking, “Why does this have to happen? My mind is so alert, there is so much more I could do.” For that would be a denial of reality, which would undercut your mind’s connection to reality, such connection being necessary for any further success of any future venture.
If you’re an archeologist, but can no longer dig among the ruins, you may have to devote more time to writing about the facts of your profession. And if you become so weak that you can do nothing, then with a last noble act you may choose to end your life. At that point it would not serve you to think, “I am still mentally alert, I still have a powerful intellect. Why can’t I live forever? I could do so much more.” But, you see, there is always more, and at the end of another hundred years of extr4a living you would be thinking the same thing—“There is so much more I could do.”
If you have lived a full life, if you have made yourself proud and happy by what you have done, and now your physical powers are waning, then there is no more for you. There is more for someone else—there will always be more— but “more” is no longer your concern. You have it all, and you are the end of your own, special, one-of-a-kind life. Nothing can be more than that.