A 23 year old in the 17th century would expect to live nearly as long as a 23 year old today- the depressed life expectancy statistics are largely due to the fact that 3 out of 4 children died before they hit the age of five.

http://en.wikipedia.org/wiki/Life_expectancy

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.

You don't have to guess if you're willing to move a little forward in time:

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.

Rome wasn't much worse off- a 20 year old would expect to hit 54.

Here's hoping for the hockey stick.

http://www.utexas.edu/depts/classics/documents/Life.html

England in 1850 was already industrialized.

I suspect there were significant differences in life expectancy between the 17th century and 1850.