What you say about life expectancy at birth is true, but it's also true that people who made it to adulthood didn't live nearly as long. I'd guess the average life expectancy of a 23 year old in 17th century England was under 60.
http://www.infoplease.com/ipa/A0005140.html
So the average 20 year old in 1850 could expect to live an additional 40 years, to roughly age 60. The curve started flattening after WW1 and continued doing so until around the 70s and 80s.
Here's hoping for the hockey stick.
I suspect there were significant differences in life expectancy between the 17th century and 1850.