By the time the baby boomers reached adulthood in the 1960s, most northern and western European countries had caught up with and surpassed the United States. Young adults in Japan and other prosperous Asian countries now stand nearly as tall as Americans do.
Now don't get all scared and start wondering if there's something in the water. This is the average height across the entire population of Americans compared to average heights around the world. It's no wonder we've lost our edge since our height average is going to be affected by immigrants from third-world nations, something Japan doesn't have to worry about since they're so xenophobic they barely tolerate other Asians. A lot of nations' living standards have also increased in the past 50 years, including pretty much all of Europe, so coupled with their ethnic homogeneity, again, it's no surprise their average height has increased that much.
The study itself says:
Komlos' most recent data indicate a small uptick in the heights of white Americans born between 1975 and 1983, a suggestion that the gap may finally be closing. But there has been no similar increase among blacks, a suggestion that inequality may indeed play a significant role in the height gap.
In another recent paper, Komlos and Lauderdale also found height inequality between American urbanites and residents of suburbs and rural areas. In Kansas, for example, white males are about as tall as their European peers; it's big cities like New York, where men are about 1.75 inches shorter than that, that drag America's average down.
One of the important points to remember is that height correlates very well to levels of prenatal care and nutrition in early childhood. In short, what Komlos says this study is telling us is that American children aren't doing well enough, on average. What it's also telling us is that there are two populations; those who are getting good nutrition and care when young, and those who aren't. That's a gap that shouldn't exist.