You can see why ancient life expectancies were low for yourself: figure out the average age at death of the following group of people:
Grandma Julia: 85 (from a stroke)
Grandpa Lucius: 45 (from diabetes)
Mom: 56 (from cancer)
Dad: 45 (from a heart attack)
Maria: 8 (pneumonia after a bad cold)
Favilla: 3 (dysentery on top of malaria)
Marcus: six months (same pneumonia after same bad cold)
Quintus: three months (digestive problems)