Subtracting your current age from your life expectancy gives the average number of years you can expect to live. Ancient people had a very short life expectancy so very few reached old age.
Don’t believe that!
Life expectancy in the United States is currently about 76 for men and 81 for women. So, a 70-year-old U.S. man can expect to live, on average, another six years, right? Wrong.
“Life expectancy” actually means “life expectancy at birth.” All male babies born in the U.S. today can expect to live to an average age of 76, given current medical treatments, lifestyles, etc. But, some will die at age five, age 16, age 97… 76 is just the average of all of those lifespans. Imagine two babies born on the same day – one dies just after birth and the second lives to be 100. The average age of those two individuals is 50, yet if you are the lucky one that doesn’t die young, you will live to be much, much older than the average age of 50.
If you make it to, say, one-year-old, you are among the group that has avoided infant mortality. Your life expectancy is now greater than 76 because those very early deaths are no longer averaged in. If you make it to age twenty-one, you have avoided teen suicide and car accidents, increasing your expected lifespan even further. And so it goes each year of your life – otherwise, once you pass age 76 (81 if you’re female), you would have a negative number of years left?
Mortality tables provide the answer here. These tables are used by life insurance companies to calculate insurance rates. A 76-year-old male would expect to live, on average, another 10 years. If he’s still alive 10 years later, at age 86, he can expect five more years.
This basic misunderstanding of life expectancy also distorts our impression of prior generations. What if I told you that the average male life expectancy in 1907 was 45 years? You might imagine a society where anyone reaching their 40s was considered elderly, and few reached 50. The truth is that the average lifespan of this era was skewed by a very high infant mortality rate of 10%. That’s almost 14 times higher than it is today! Lifespans were shorter back then, but people weren’t keeling over in their mid-40s; many lived well into old age.