The number of people dying will increase two score and ten years [2] after a baby boom [1] like the United States post-war baby boom.
[1] https://en.wikipedia.org/wiki/Baby_boom
[2] https://www.biblegateway.com/passage/?search=Psalm%2090%3A10...
[1] https://en.wikipedia.org/wiki/Baby_boom
[2] https://www.biblegateway.com/passage/?search=Psalm%2090%3A10...
You can also look at other samples like the Founding Fathers of the US. Most lived past 70, and some even lived beyond 90. The only people who died before 60 were Alexander Hamilton, killed in a duel and John Hancock, died of gout - which can be caused by things including excessive drinking. There was indeed a very low life expectancy in the past, but it was 100% due to infant mortality. If one child dies in infancy, and the other lives to 80, then you have a life expectancy at birth of 40.
That the Bible itself also noted this, is extremely interesting. Not even from a religious perspective, but from a historic one. It's really interesting to think about. As an adult today, you have a similar life expectancy to an (upper class) adult from thousands of years ago. There are many implications that one can apply to modern life. For instance it's safe to say that their dietary/lifestyle choices are very much worth considering, if not emulating.
A better link would have been https://www.oed.com/dictionary/threescore_adj?tl=true#:~:tex....