Your factoids from the last half of the 19th Century persisted with some change into the early 20th Century in the former Old West, in my paternal grandparents’ case in KS. My grandmother began having children at age 20 in 1902 and concluded at age 40, bearing 10 children, 9 living to adulthood. So her progeny mostly beat the infant mortality stats. Her husband died at age 51 of abdominal sepsis from a burst appendix. She died 8 years later at age 50 of cervical cancer.
The infant mortality rates impacted longevity, but in a nuanced way. If average lifespan was, say, 40, anyone who survived beyond infancy actually had as much chance of living to 70 as much later. The high infant mortality rates skewed longevity, which was an average.