maybe some just don't want to live that long. does everyone need to live as long as possible? you live your youth, and maybe that is enough for some.
if you start to hurt every day, get diagnosed with age-related disease, your cognition starts to deteriorate, etc, you might just decide "hey, i lived enough. i don't want to go through years of feeling like this."
Much easier said than done. Ending your life is the last decision you'd make.
That's why Mishima topped himself at the age of 45: as a physical-conditioning buff and gay man, he came to a firsthand realization that it was all downhill from there. What you posted is very similar in spirit to the opening of his last novel.
I think about this increasingly often. I have accomplished much, and have seen much of the world. I have also lived through gruesome deaths in my family and have seen how aging in America, under American healthcare looks.
I retired in 2023, not because I could, but because I couldn't not. I'm almost 50. My parents died in their early 60s. I have some of their health problems, and some of my own. I also destroyed my back in a motorcycle accident many years ago and live with constant pain.
My favorite things to do, which bring my peace and joy, involve me sitting in a chair, which hurts my back. In my youth I rode my bicycle avidly, which now hurts my back. I love gardening, and if I do it too much, it hurts my back.
It became increasingly clear to me that my "purpose" in this country is to work until I die. I know that there are other countries, and other kinds of lives, but they are not where my wife and friends and life are.
I am happy, relatively healthy, and love life. I would like to live forever if I could. I know I could fill every minute of it with things I find interesting, even if it was just reading every single book.
But I've seen what American life looks like in your 60s and older, and it's not fucking pretty for most people.