A war could do it. Young people dieing will pull an average life expectancy down a lot.
Also check water treatment plants.
I do not know if shortages in medical materials or pharmaceuticals is linked in the game. A real world drug shortage would lower real life expectancy.
In real world demographics you have two different numbers. For example, the life expectancy of a human born in the United States in January 16, 2009 is not the same as the life expectancy of humans living in the united states. A newborn can die before reaching age 5. Everyone currently over age 5 is 100% guaranteed to live to be older than age five. If you have a baby boom reaching old age you will see both the average age and life expectancy go up. If you have a baby boom currently getting born the life expectancy should drop.