Georg von Rosen - Oden som vandringsman, 1886 (Odin, the Wanderer)
It seems to be a common belief that the current generation of people below the age of 20(ish) will be the first generation to live a shorter lifespan than their parents. It's often paired with the belief that every generation BEFORE this generation has lived a slightly longer time than the one before them (with a few exceptions due to massive wars).
So much so that I've heard people say that people died of old age in Ancient Egypt while in their mid-20s. And that you were "ancient" if you made it to 30. Now, obviously this can't be true for all of human history, or else people would have been dying of "old age" while they were still infants. [*]
But how much younger was "old age" back in "the day"?
A lot of the confusion, I think, comes from a confusion between the differences of "life expectancy" and "lifespan." Life expectancy is the noun used to describe the average period that a person may expect to live. It's calculated by taking the ages that everyone in a population was at their death, and then dividing it by the number of people.
It works better if I give a small example. Let's say there is a village of 10 people (it's a very small village). The ages of these people when they died were:
- 2 years
- 10 years
- 45 years
- 49 years
- 20 years
- 34 years
- 1 year
- 15 years
- 31 years
- 70 years
If you take those then people and add their ages at death together, you get a combined 277 years. Then you divide by 10 (the number of people) to figure out the average -- 27.7 years. So the average life expectancy would be just under 28 years old.
But nobody was actually that age when they died. Instead a lot died when they were kids, a lot died when they were middle aged, and one person made it into their seventh decade.
Now I completely made up those numbers, because in real life it's a little more complicated. Men tended to live longer than women, because women died in childbirth a lot. Children also had a high mortality rate.
For a male child in the middle ages, if he could survive to the age of 21, he could reasonably expect to live another forty years. [**] Childhood was the most dangerous time in a man's life. If he could get though that, he was well on his way to making it to our modern retirement age. If a 21 year old woman didn't have children in the rest of her life, she could also reasonably expect to live six or more decades.
Childbirth was a woman's biggest risk, historically, with the average of 1 in 100 deliveries resulting in the death of the mother (though this number has changed a lot in time, rising from as high as 40 per 100 of all births down to 24 per 100,000). [***] Since a woman could realistically have multiple pregnancies, this was a risk that was more or less constant through her life until menopause -- the average woman in the early modern era had 7-8 life births in her lifespan, each one brought risk of death [****] (an example is Queen Jane of England, the wife of Henry VIII -- she had the best medical care available and died giving birth to her first child).
You can see how this number is suddenly a lot more complex than "people in year X lived to be Y years old." Realistically, if you could survive childhood, you had the right to look forward to a lifespan long enough to get you membership in the AARP today -- as long as you weren't a woman, at which point each delivery was something like a time bomb.