Life expectancy in the United States has steadily increased over the course of the last century, but most Americans do not live to be 85. Today, there are roughly 6 million Americans 85 and older, just 1.9% of the U.S. population. While octogenarians make up a relatively small share of the total population, some parts of America have significantly higher shares.

24/7 Wall St. reviewed population figures from the U.S. Census Bureau’s American Community Survey to determine the county in every state where the largest share of residents are at least 85 years old.

For both historical and natural reasons, there are not many Americans over the age of 85. The economic stress of the Great Depression — which took place during the 1930s — led to a decline in fertility that only picked up at the conclusion of World War II. A baby boy born in 1930 was expected to live 58 years.

Living more than 85 years is no easy feat. Over the course of such a long life, many of these individuals likely had access to medical care, healthy lifestyles, and were lucky enough to avoid accidents. Environmental conditions and other factors also play some role in an individual’s life span.

7. Middlesex County, Connecticut

> Pct. of population 85 and over: 2.7%
> Pct. of state population 85 and over: 2.4%
> Median age: 39.6 years
> Average annual retirement income: $24,983

It might be expected that the communities with higher shares of older residents are retirement destinations. However, the counties with the highest shares of adults 85 and older are in places like Montana, Nebraska, and Kansas, rather than in states where tens of thousands of retired Americans move each year, such as Florida, Arizona, and California.

Rather than being popular destinations for retirees to move, counties with the highest shares of older residents may simply have few young residents living in them. Young Americans are continually moving to urban areas and likely leaving many of the counties on this list. Older Americans, by contrast, tend to age in place. The outbound immigration of an area’s younger residents may ultimately leave a higher share of adults 85 and over.

While life expectancy tends to be higher in urban areas, many counties with the highest share of residents 85 and over are rural areas. Counties with the highest shares of octogenarians, nonagenarians, centenarians and supercentenarians are clustered in the rural Midwest — many in Montana, South Dakota, and North Dakota.

Nationwide, the population density is 87 people per square mile. In the five counties with the largest shares of residents 85 and older, the density is less than four people per square mile. Within each state, counties with lower population densities tend to have larger shares of older Americans. In Treasure County, Montana, where one in 10 residents is 85 years or older, there are just 0.7 residents per square mile.

To determine the oldest county in each state, 24/7 Wall St. reviewed population figures from the U.S. Census Bureau’s American Community Survey (ACS). Median age came from the ACS as well. The average annual retirement income was calculated from the aggregate retirement income received in the last 12 months and the number of households receiving retirement income in a given county, also from the ACS.

Click here to see the oldest county in every state.