I think most of us spend a certain portion of our lives thinking about or imagining what life will be like when we are old. The definition of what is "old" certainly changes, particularly as some of those previous benchmarks get closer! Some of us spend more time than others in this activity, and we all have different experiences to draw from that inform how we imagine that will look. But most of us do picture ourselves reaching some sort of state of decrepitude at some point in our lives.
The wonderful and delightful Terry Pratchett discusses this much more eloquently than I could ever hope to in this Guardian post discussing practical solutions in the UK.