Unless you want to argue that it was thanks to her study of radium that Marie Curie didn't die as a newborn, "average life expectancy" is not the correct statistic to apply.
Children died, a lot. You're confusing the definition of average.
The thing is that with all of our medicine we haven't much increased the age at which people die from being old. Plenty of 75 year olds back then, but they almost all had a sibling that died of a childhood illness.
Mostly what medicine has done is vastly increased your chances of living to adulthood.
