America grew a little bit older in the past year: Our national median age rose by one-10th of a year, from 37.5 to 37.6 years, according to new data from the U.S. Census Bureau. But that tiny shift masks a considerable amount of variation and churn at the county level. More than 40 years -- half of a lifetime -- separates the median ages of our oldest and youngest counties. And while most counties grew older, nearly one-fifth of them saw their median age decline over the past year, in some cases by a year or more.

Many of the nation's youngest counties are in the Southwest; Utah, Southern California, and parts of Texas and Arizona stand out as places where the median age is in the low 30s. Utah in particular has long held the title as the nation's youngest state. More than half of Utah residents are Mormon, and the church's emphasis on the importance of family means that Utah's fertility rate has consistently stood above the national average.

On the other hand, three of the nation's 10 oldest counties are in Florida. But in addition to being a retiree haven, the state is home to youthful urban areas like Miami, as well as a large immigrant population. These factors bring Florida's median age below that of the three oldest states in the nation -- Vermont, New Hampshire and Maine, all in northern New England. With little immigration to speak of and not much in the way of large urban areas, that region's younger residents are moving out as the rest of the population ages in place.

Looking at the change in median age since 2012 (use the toggle buttons on the map above), four of the 10 counties showing the largest decrease in age are in the Dakotas. Like Benjamin Button they're becoming more youthful with the passage of time. Why? One word: shale. The energy boom in the high plains may well be the biggest demographic story of our generation. The influx of young, mostly male workers to the Bakken shale region is affecting everything from the region's time use to its economic development.

Williams County, in North Dakota's northwest corner, is the epicenter of the state's shale boom. Its population surged by 10.7 percent from 2012 to 2013, the biggest jump among any county in the nation. Its median age fell by 1.6 years over the same period. If the Benjamin Button analogy holds true and that trend continues, everyone in Williams County will be an infant by 2034! (Note: That was a joke.)

Looking at the map you'll also notice a streak of green counties running down the center of the country. William Frey, a demographer at the Brookings Institution, found this surprising. "You'd expect those places to be getting older," he told me. "They're not attracting a lot of young people." Immigration is likely driving the declining age in these counties.

There's not as much of a discernible pattern to which counties are getting older the fastest; they're spread all over the country. For more on the state-level demographics -- with some great state-level maps -- read Niraj Chokshi's take in GovBeat.