I sometimes look at it mathematically. I don't know if I am doing this just because it seems to give a pro-CTB answer. But here goes.
I've seen my parents age and die, so I know a little bit about my genetics and what can be expected. I think my unaffected life span would probably be about 90 years, plus or minus. It's well known that loneliness is a great killer and can age you years. There are reasons to think I could live a little longer--each generation has a slightly longer lifespan, I'm in reasonably OK health etc.; and there are reasons to think I could live a little shorter--the loneliness issue, I drink a bit too much, etc. If the next 20 years are truly and blissfully happy, I think I can live beyond 90. If they are miserable and lonely, I think I will not live to 90.
In either case, I expect my mind and body to start deteriorating at 80, and I expect this deterioration to accelerate at age 85. Whether I am alive or dead, by my mid-80's my life will likely be pretty much not be worth living. When my mom broke her hip, she went to a rehab center that was mostly elderly folks. What a depressing place. Old people just sitting and staring and waiting to die. There's no interest in life and no excitement and no pleasure to be garnered. A day in the life of an old person is basically sleep, wake up, eat something, sit around, go back to sleep, wake up, eat a little more, go back to sleep. There's no joy and no pleasure.
So, I figure if you look at the arc of life, 80 is a good time to go. Any time after that may well be a negative. Of course, some people are exceptions and they are able to live active, fruitful and engaged lives for much longer. But that's a rarity.
So, if I am pushing 60 soon, that means I've lived 75% of the good part of my chronological life as measured in years. If you measure it not in years but by other yardsticks--happiness, accomplishment, the sense of wonder over new things; or other tangible life events--graduating from high school, getting into college, your first girlfriend, your first job, your first promotion, your first business success, your first home, your first wife, second wife, first kid, second kid, watching my kid get into school and college and all that, then I've really lived 90% or more of my productive life. What's left?
My 24 yo is an adult and we have a fraught relationship since her mother poisoned her against me. I don't see her tending to me in my old age. My 13 yo it would be nice to see her grow to adulthood, but she will be fine with her mother. She will always be a lot closer with her mother and that's a lot of the reason why men need women--women keep the family together. Without women, men just drift off into loneliness.