The real issue is child morbidity. There are plenty of old people in most poor countries but if 10+% of the population dies before age 5 the average life expectancy get's far shorter.
For comparison US death rate in the 0 - 5 range is really low. http://en.wikipedia.org/wiki/File:Causes_of_death_by_age_gro...
PS: Nutrition is huge: According to Jean Ziegler (the United Nations Special Rapporteur on the Right to Food for 2000 to March 2008), mortality due to malnutrition accounted for 58% of the total mortality in 2006: "In the world, approximately 62 millions people, all causes of death combined, die each year. In 2006, more than 36 millions died of hunger or diseases due to deficiencies in micronutrients"[4].