As I mentioned a few weeks ago (link), overall U.S. life expectancy fell slightly in 2015. We should be careful about reading too much into a small one-year decline (it also fell in 1980 and 1993), but we shouldn't ignore it either.
In addition, for a couple of decades life expectancy has been stable or declining among subgroups of Americans even as it has continued to advance for others. More surprising, the subgroups in question are mainly subsets of whites, particularly less-educated whites. It's worth emphasizing that white Americans still tend to live longer (in fact, quite a lot longer) than many nonwhites. But while nonwhites are gradually doing better, whites are not, and some subsets of white Americans are even losing ground. There's an on-going debate about why this is, and abuse of dangerous drugs such as meth and prescription opioids may be a significant contributing factor but not a full explanation.
For a long while U.S. life expectancy has been lower than in many other developed countries, but that's more or less a constant. There are a number of reasons for that difference, ranging from universal or near-universal health care coverage in other countries to our relatively higher rates of drug abuse, accidental death, and crime.
As this article from The Dallas Morning News examines, one explanation for the difference is the relative level of gun violence (including suicide by gun) in the Unites States.
No matter what you think about firearms regulations, this article is worth reading.