A decades-long trend of rising life expectancy in the U.S. could be ending: It declined last year and it is no better than it was four years ago. In most of the years since World War II, life expectancy in the U.S. has inched up, thanks to medical advances, public health campaigns and better nutrition and education. But last year it slipped, an exceedingly rare event in a year that did not include a major disease outbreak. Other one-year declines occurred in 1993, when the nation was in the throes of the AIDS epidemic, and 1980, the result of an...