The average American is getting older as baby boomers hit retirement age and natural births decrease, according to new data from the U.S. Census Bureau.

The median American was 38.2 years old in 2018, the Census Bureau said, up from 37.2 years in 2010. The median age has risen particularly fast in Northeastern states, which have grown at a slower pace as boomers leave their child-bearing years and younger residents move to warmer climates.

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Maine, New Hampshire and Vermont are the oldest states in the nation. The median Mainer is nearly 45 years old, 2.2 years older than the median eight years ago. The median age in New Hampshire, Vermont, West Virginia and Florida are all north of 42 years old, numbers that have grown faster than the national average.

Only one state, North Dakota, has seen its median age drop since the beginning of the decade, the figures show, a reflection of that state’s population boom as the fracking industry took off at the beginning of the decade. The median North Dakotan is 35.2 years old, nearly two years younger than eight years ago.

Utah is the youngest state in the union, with a median age of 31 years. The median residents of Texas, Alaska and the District of Columbia are all under 35 years old.

“The nation is aging,” said Luke Rogers, who heads the Census Bureau’s Population Estimates Branch. “This aging is driven in large part by baby boomers crossing over the 65-year-old mark.”

The number of Americans who have crossed into retirement age is growing rapidly, the new figures show. In 2016, 16 percent of the U.S. population was more than 65 years old, up 30 percent since 2010.

At the same time, the number of Americans who are under the age of 18 has fallen by 1.1 percent, a reflection of an ongoing phenomenon demographers call the baby bust. Women of childbearing age are having fewer children, the data show, and they are waiting later in life to have those children than earlier generations did.

Population growth has slowed most significantly among white Americans in recent decades, but the new Census data show members of other races are growing older too, another sign that the baby bust crosses demographic boundaries. The median African American is 1.4 years older than they were in 2010, the median Hispanic American is 2.2 years older and the median Asian American is 1.7 years older.

Still, the nation is becoming more diverse, and that diversity is increasing at a faster rate than it had in the past. The white population in the United States increased by just 220,000 people between 2010 and 2018, while the number of African Americans grew by about 3 million, and the number of Hispanic Americans grew by more than 9 million.

Demographers project the future of American population growth is likely to be driven more by immigration than by births. In 2017, the country experienced a net natural increase of 1.4 million people and an increase of 1 million through international migration. By 2030, the Census Bureau projects international migration will exceed natural increases by about 100,000.