The average serf might have lived to 20, but the old guy was still probably about 70.

Babies and little kids are doing a lot better but the life expectancy for guys like me (51 as of Friday) hasn't changed much in the last 500 years.

Also, the line is "age and treachery always beat youth and skill". But with age, we forget the difference between skill and treachery, so there is not much difference.