On the other hand, we have John Milton lamenting his 23rd birthday (http://www.luminarium.org/sevenlit/milton/sonnet7.htm):

  HOW soon hath time, the subtle thief of youth,
    Stolen on his wing my three and twentieth year!
    My hasting days fly on with full career,
    But my late spring no bud or blossom sheweth.

  Perhaps my semblance might deceive the truth,
    That I to manhood am arrived so near,
    And inward ripeness doth much less appear
    That some more timely happy spirits indueth.

  Yet be it less or more, or soon or slow,
    It shall be still in strictest measure even
    To that same lot however mean or high,

  Toward which time leads me and the will of heaven.
    All is, if I have grace to use it so,
    As ever in my great taskmaster's eye.

23 could be a bona fide mid-life crisis in the 17th century :)

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.

"Most men lead lives of quiet desperation and go to the grave with the song still in them."

Henry David Thoreau

Youth is wasted on the young. -- Mark Twain