The book I read also went into lifespans of people in England before that time.
Lifespans have not changed much until very recently. As I mentioned, the bible mentioned 3 score and 10. However, average life expectancy has always been lower than now, everywhere, and was very low for the most part. You mentioned that if you survived childhood and pregnancy, you could live a long life. Well, ya, but that is not what I was referring to.