> Life expectancy of an adult[1] at that time was only about 48 years old

Where did you see that? I found this article https://pmc.ncbi.nlm.nih.gov/articles/PMC2885717/#:~:text=20... which says that in 1790, a 20 year old American white male could expect to live to his early 60s. Interestingly, life expectancy in the US actually went down during the 19th century due to urbanization. One thing you have to watch out when you're looking at numbers like this is that they catalog remaining life expectancy. For example, the chart says "41.4" but you have to add 20 to get the total life expectancy since it's remaining lifespan at age 20.