The world population has quadrupled in my lifetime, so I would be willing to believe the old bit about “more people are alive now than have ever died.” But it’s bunk. Estimated count of all people ever is 100 billion. There weren’t that many people in the past but our species goes back 50,000 years and that makes it up.
The book is over half a century old now, so the numbers may be a bit off, but this sort of conversation always reminds me of this quote
"Behind every man now alive stand thirty ghosts, for that is the ratio by which the dead outnumber the living. Since the dawn of time, roughly a hundred billion human beings have walked the planet Earth."
Men make their own history, but they do not make it as they please; they do not make it under self-selected circumstances, but under circumstances existing already, given and transmitted from the past. The tradition of all dead generations weighs like a nightmare on the brains of the living.
We're even older than that! There is compelling evidence that Homo Sapiens has existed for 400k years, and there's unprovocative evidence that we've been around for 250k years or so.
Which is why the average life expectancy was in the 30s forever. If you made it past childhood you were likely to make it to old age, but the infant mortality rate was through the roof which brought the average down to less than half of what it is today. People regularly lived into their 70s-80s before, but the average of 30 years makes people think that's all the longer people normally lived.
Even if you look at monarchs (with relatively good living standards) who died of natural causes, those who make it to their 70s and certainly their 80s are pretty rare. Doesn’t mean the ‘everyone died in their 30s’ thing is true, but I’d say making it to your 50s and maybe 60s would be a more reasonable expectation
Although your general sentiment is right, even adult life expectancy has gone up dramatically. For instance, for 20 year olds he life expectancy was about 60 in 1850, but now is around 82.