Why do people like to use average life expectancy? "Average life expectancy was 45 years old at the turn of the century."

That provides very little information about the distribution of lifetimes and provides a biased view when compared to the 85-year-olds mentioned in the next sentence. Infant/child mortality skews the average a lot. You could have everyone living to 90, if they survive to 5, and still have an average of 45. War and other disasters can also change the average in ways that have little meaning for the medical programs in question.