This study suggests that from DNA evidence that natural human lifespan is 38 years old, the same age that was the life expectancy during Wunderlich's 1851 survey.

https://elifesciences.org/articles/49555

Additionally, our heating systems were vastly less sophisticated, and we were outdoors more.

Together I think that these suggest that we get a sort of genetic gift of our bodies in our prime, but the growth program kind of peters out in our late 30's. After that it's a genetic maintenance program, with which, in concert with modern society, we have a good chance to keep the machine going for quite a long time.