But if you eliminate life expectancy at birth, you'll notice that there hasn't been a drastic improvement. It's been discussed on HN previously, and I think adult life expectancy has only improved by less than 20 years over the last several hundred.
(Someone with sources feel free to correct me).
As an example, in the 20th Century, life expectancy at birth rose from about 40 to 77 years. But for 20 year olds, it also went from 34 to 59 additional years.
Page 52 Men in 1900 life expectancy at 20: 42.48
Page 54 Women in 1900 life expectancy at 20: 43.6
The Social Security life table for 2016 lists 56.91 years at 20 a gain of ~13.9 years in the last 116. https://www.ssa.gov/oact/STATS/table4c6.html