That wasn’t the average lifespan of people who reached adulthood. Infant mortality was simply higher.
Also my grandmother mentioned that when she was young girl just a 100 years ago old people were absolutely broken. Today a lot of people spend their sixties and seventies in very good health.
Also I remember reading about grave site on a bluff in California that was eroding away. Contained the graves of sailors that died when a ship sank in the 1880's. Before reburying them higher up on the bluff they looked over their bones. They weren't in very good health despite being in their 20-40s.