What scares you? Mortality, or being able to keep being useful, or something else? (I'm not being flippant: I live next to a graveyard which reminds me that I'm not getting out of the first of those!)
I guess its a combination of Mortality and lack of independence/increasing dependence, while bodily functions slowly get less and less effective. I am very afraid of the idea of fading away. Perhaps I would be more comfortable if, e.g. I knew that my end would come at a very specific day and that I would remain healthy until then and the poof!
Then you should know the joke about the difference between an English actuary and a Russian actuary. The former tells you your life expectancy ie an age you might expect to die, whereas the latter gives you a date, time and place.
I suspect I'm somewhat older than you, and faults are already accumulating and have been significantly so for 20 years or more. I can see how it may eventually become too tiresome to carry on if some of the breakage can't be fixed. But there are often compensations...