Well one answer is that they didn't. Infection was a major cause of death. When you see a brutal dentist scene in a historical film it gives shivers, the missing context is that the patients life was probably being saved.
And not like they were on life support when hitting over 100, they were out in the fucking field working.
But yeah, eventually this will disappear and all we have is a book saying that 100 years ago average life span was 30 years so we are doing great increasing it to 60 :D