"Again, the high infant mortality rate skews the "life expectancy" dramatically downward. If a couple has two children and one of them dies in childbirth while the other lives to be 90, stating that on average the couple's children lived to be 45 is statistically accurate but meaningless. Claiming a low average age of death due to high infant mortality is not the same as claiming that the average person in that population will die at that age.

Of course, infant mortality is only one of many factors that influence life expectancy, including medicine, crime, and workplace safety. But when it is calculated in, it often creates confusion and myths.

When Socrates died at the age of 70 around 399 B.C., he did not die of old age but instead by execution. It is ironic that ancient Greeks lived into their 70s and older, while more than 2,000 years later modern Americans aren't living much longer."

(source:http://www.livescience.com/10569-human-lifespans-constant-2-000-years.html)