We only really live about 6 and a half years longer than we would prior to the industrial revolution on average. This relatively modest number is obscured somewhat by overall life expectancy averages which are buoyed by drastically reduced infant mortality.

In any case, the political/economic response has been to move up the age of retirement, which isn't helpful to the long term knee health of the hypothetical farmer worker here.

  We only really live about 6 and a half years longer than we 
  would prior to the industrial revolution on average. This 
  relatively modest number is obscured somewhat by overall  
  life expectancy averages which are buoyed by drastically  
  reduced infant mortality.

I haven't heard this before - where are you getting your data?

Life expectancy numbers should be quoted at 'life expectancy at 10 years old' if you want to filter out child mortality in the past.

Certain economic actors don't want it to catch up.

Yup. The right-wing Australian government have just moved the retirement age from 65 to 70. What a joke.