The low life expectancy in 1860 was primarily because so many children died of childhood diseases that can be prevented now. Who today knows anyone who died of diptheria or whooping cough? I remember when the polio vaccine came out; the line to get the shots extended all across the university campus! (We had just gone through an outbreak that killed several popular students and left others paralyzed.)

People who survived childhood and major accidents (and, for women, childbirth) tended to live as long as they do now.

Of the approximately 750,000 who died in the Civil War, it is estimated that half died of disease. That was still the dark ages of medical and public health knowledge, which began to advance only later in the 19th century.