In the 1800s, America’s young men went West for gold. Today, younger folks are heading to the heartland for oil and gas.

As a result, while the rest of America is getting older, there’s a chunk of the country that’s getting younger, according to census data released today.

The U.S. Census Bureau’s data shows the median age is coming down in Oklahoma, Montana, North and South Dakota and Wyoming — states where energy is being extracted from the earth at a record pace in part through America’s new wave of hydraulic fracturing, or “fracking.”

“We’re seeing the demographic impact of two booms,” U.S. Census Bureau director John Thompson said in a statement. “The population in the Great Plains energy boom states is becoming younger and more male as workers move in seeking employment in the oil and gas industry, while the U.S. as a whole continues to age as the youngest of the baby boom generation enters their 50s.”

In 2013, the 65-and-older population hit 44.7 million, a 3.6-percent spike from 2012. Sumter County, Fla., is the county with the highest median age: 65.5. The county with the lowest was Madison, Idaho, at 23.1.

The changes in the states that are getting younger may seem small: The median age in Montana dropped from 39.962 to 39.898; North Dakota from 35.881 to 35.270; Oklahoma from 36.233 to 36.226; South Dakota from 36.841 to 36.818; and Wyoming from 36.854 to 36.828. Alaska dropped from 33.606 to 33.246 and Hawaii dropped from 38.138 to 37.963.

But that’s just the change from 2012 to 2013, a time when nearly every other state in America was aging. Over time, these changes are may prove to be more dramatic.

Even though North Dakota as a whole got younger by only 0.6 years, the state’s Williams County — the center of the Bakken shale energy boom — led the nation with a decline of 1.6 years. The data shows men are flocking to this area — now making up 51 percent of North Dakota’s population. Experts say the surge of youngsters is fueled by work they can get there.

Between 1950 and 2010, North Dakota’s population growth was one of the most sluggish in the country, according to the Population Reference Bureau (PRB). But in the past several years, it has become America’s fastest-growing state with a 7.6-percent increase in its population.

In North Dakota and many other parts of the Midwest and Great Plains, these population spikes were preceded by a staggering increase in shale oil production, PRB reported. Earlier this year, the U.S. Census Bureau reported that employment in oil and gas extraction, mining and quarrying jumped 23 percent from 2007 to 2012, making the business one of the fastest-growing industries in the United States.

Much of that growth was concentrated in the Bakken Shale — an oil-rich rock formation that has made North Dakota the second-biggest oil producer in the country after Texas.