That link is to a racist and xenophobic blog. I would start by finding some better sources, then proceeding with research from there.

I'm not clicking any link with slavery-myths in the URL, but infant mortality was very high in the 19th century. In New York in 1840, for example, the mortality rate before the age of 5 was 50%. So half of all babies born died before the age of 5. I don't have life expectancy figures to hand but in places with high child mortality, your life expectancy goes up as you survive childhood. Life expectancy at birth may well have been 24. But life expectancy at 5 might have been 45, life expectancy at 10 might have been 60 and life expectancy at 18 might have been 70.

Source: Meckel, Save the Babies: American Public Health Reform and the Prevention of Infant Mortality 1850-1929 (1990) University of Michigan Press, p18