A major demographic trend in the past 40 years has been decline of the Heartland and the Rust Belt. Between 1970 and 2010, the U.S. added 100 million more people — but not in these areas of the country, which have seen sluggish population growth, and in some areas, population loss.

As the nation continues to grow, it’s leaving behind states like Nebraska, which only grew 15 percent between 1970 and 2010.

One group to keep an eye on is young workers who are at the age to be starting families. As this chart from the Congressional Research Service shows, births still account for much of the nation’s population growth, more so than immigration or the declining death rate. Having a healthy population of people between the ages of 25-34 should be a good sign for a region, indicating that there is enough opportunity for people to want to settle down there.

The trends for 25-34 year-olds in the past 40 years are pretty much the same, though. Places that have seen overall population loss have seen similar declines in the numbers of young workers.

But what if we were interested in the age of the people still living in a particular region? Here’s where things get surprising. First, some background: In 1970, 25-34 year-olds were 12.3 percent of the U.S. population. In 2010, they were 13.6 percent. Of course, some places saw the fraction of young people increase. For example, San Francisco County saw its fraction of young workers increase. That is, 25-34 year olds made up about 15 percent of the San Francisco population in 1970, and 21 percent in 2010.

Other places, like Roanoke County in Virginia, saw the fraction of young people decrease. They were 14 percent of the population in 1970, and only 10 percent in 2010.

This is a map of where the share of young workers grew between 1970–2010, and where it shrunk.

In this picture, the Midwest seems to be much healthier. Despite the huge overall population losses in the past 40 years, the balance of young workers has remained more or less the same. Immigration in the past decade has helped to add youthful workers to the region.

In contrast, the Rust Belt and Appalachia saw big declines in the fraction of people 25-34, accompanied by increases in the median age. The story here is that young people have moved out, leaving the older residents behind, and without much immigration to plug the hole. The great challenge for these regions will be to bring back young workers, whether they’re native-born or immigrants. Because if nobody’s starting families in these places, the population will only continue to dwindle.