The following is a snippet that my 79 year-old mother wrote in recent letter:
... I am so thankful to have very good health and energy. Of course, I do work at that by maintaining excellent nutrition and exercise. I try to walk six miles (very brisk) at least 5-6 times a week. When the weather is bad, I go up to the penthouse to use the treadmill (which I don't like very much), or walk the stairs here in the building (there are 13 floors), or I can always go over to the gym ...
Note that when she says "walk the stairs," it means that she walks up all 12 flights from the ground floor to the penthouse on the 13th floor. In previous correspondence, she wrote that when she is not feeling well, she just walks up the first 5 flights of stairs and then takes the elevator to her apartment on the 11th floor. (Of course, being the empathetic son that I am, I chastise her for being so lazy at times ... "I don't care if you have the flu. Suck it up, lady. Next time I want all 12 flights out of you.")
While people experience a variety of physical (and mental) circumstances, many beyond their control, I believe much of the aging process depends on: 1) how you perceive yourself, and 2) how you care for your body. While this may sound trite, and it is not meant to minimize the real physical problems some people face, you are only as old as you act and feel.
In my opinion, if you think you are too old to perform a given activity, then you probably are too old. So do not give up. Change your self perception. Do not use age as an excuse. This is entirely different than acknowledging real physical limitations, either inherent or those caused by health issues. I will never run a 4 minute mile, but I am never going to tell myself that I am "too old" to run a 4 minute mile. And at age 54, I am fairly confident that I can still run a 5 minute mile. Yes, it first may require several months of losing weight and a lot of interval work on the track, but age will not be a factor.
You are never too old to exercise. I know many people in their 60's, 70's, and 80's who bicycle 50-100 miles. Interestingly enough, while bicycling today, I passed a retired woman from my workplace who was out running. She is ~60 years old, yet was on a run of at least 10 miles. In the hills. Nice form. Relatively fast. While this level of activity may not be for everyone, there is some form of exercise for all. But do not hesitate to think big.
I believe the OP's post is reflective. It is true. It is unlikely that any of us will live forever, at least on this earth. Take advantage of your opportunities when you can. This means ER for many of us. But I am disappointed to hear comments like, "there are only 15 good summers left." This sounds defeatist, at least to me. Yes, things may happen. That part of the body may stop working like it used to. And there may not be much that can be done to change the situation.
But I plan never to be too old. I do not believe my 79 year-old mother does either.