This isn't a fun fact really but it's something I wanted to share.
A lot of people repeat this myth that because life expectancies were 35 or whatever back in the day, that meant that there was a shortage of old people, or middle-aged people would be considered old, or other such. It's complete nonsense, and you can find tons of very old people from ancient history. But the reason for it is because something like half of everybody would die as babies, from illness/weakness. By using some figures out of a book, I calculated then that the life expectancy of a normal country before industrialization would have been at least in the 60s for those who didn't die as babies/toddlers, which is not far off from the modern day. What happened was people had less chronic illnesses coming from unhealthy modern lives, but were more likely to die in disasters and accidents.
I've used an actuarial table and made some graphs to demonstrate how this works, for modern men. See, the life expectancy as we know is the average of everybody's death age. That means, however, that your own life expectancy is not the same, because your potential age to people who died before your current age, you're comparing your potential age to people at least as old as you. That is, if you want to know how much time a person reasonably has, what you want (and this is what online calculators do) is a CONDITIONAL life expectancy, conditional at least on age and preferably on other factors as well.
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At 75 years old, for example, the conditional life expectancy is about 85, or about 10 years remaining. So you see that even somebody who reaches the life expectancy has a chunk of time remaining, on average, because we're excluding all the unfortunates who died by accident or illness rather than old age. However, the human body is designed to die around 120, so the limit of your life expectancy approaches 120 as your age approaches 120 and the limit of your years remaining approaches 0.
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Change in life expectancy from year to year is never positive (there's no point at which if you live a year, you're actually likely to live more than a year longer), but it never burns off by more than a year either (if you live through a year you had at least a year). You see that even in the modern world, babies have a noticeably higher mortality rate than others. As you near old age, though, the life expectancy change starts to get really low; that is, when you burn a year, it gets unlikely that you're going to make it another year. Put another way, people start dying faster.
There's an obvious spike in deaths from retarded teenage shenanigans, and then the rest of it looks like a logistic curve: an exponential take off followed by a logarithmic slowdown. Up faster and faster, then up slower and slower. In general people are going to keel over really fast around their 60s - 80s, but in the 90s and past if somebody makes it that far they're generally a very healthy body who can linger on quite a while.