You’re reading it wrong. Look again.
The article is NOT saying that life expectancy is 50 in the USA. It’s talking about the life expectancy that a person can have after they reach 50.
This is a statistic that factors out all causes of death prior to age 50 so that it’s a measure of actual longevity rather than a figure that is distorted by infant mortality and other early-life causes of death.
The first sentence:
According to the World Health Organization (WHO), the United States ranked 29th in the world in 2006 in life expectancy at age 50.
...is saying that America ranks 29th in the world for life expectancy, that being 50. Which isn't the case.