Please correct me if I am mistaken, but I've always thought the low life expectancies in past times was due to a much higher infant mortality rate. Once you got to your teens or so you had almost just as much a chance of a long life as today. So I wonder about authors making such arguments.
The best answer to your question is probably to make a tour of the oldest church graveyards in your area. Infant mortality is no doubt the largest factor but I would say that living to be over eighty is quite common now whereas when I was a small boy eighty was considered very, very old. Some lived to be ninety or even a hundred but that was rare indeed. Many of my aunts and uncles did not make it to 70. I am now 71 and I am about to go to the gym to work out, when it stops raining again I will finish the deck I have been rebuilding which I built almost thirty years ago. When the deck is finished I have a number of trees to cut up with a chain saw. The 71 year olds I knew growing up were mostly worn out and not able to do much of anything except sit in a chair and talk about their pain. I used to go to funerals of people who died in their sixties and hear comments about how they had lived a long, full life, now dying before 70 is considered dying young and soon dying before 80 may be considered dying young.
Remember that one main reason that social security is so underfunded is that it was originally based on the idea that most people would not live to be 65. Then again that could turn out to be the case again for some reason, I don’t make any predictions.