SEATTLE — King County is doing something that the great majority of counties in the U.S. aren't: It's getting younger.

By younger, I mean that the median age is declining. Census data released last month shows that from 2010 to 2018, the median age of King County residents dropped from 37.1 to 36.9 years.

That may not seem like much, but consider this: Of the nation's 3,142 counties, the median age declined in only 525, or 16.7% -- and most of them are in the Midwest. And among the 100 most -populous U.S. counties, King is one of only six that's getting younger.

The nation as a whole, of course, is aging. The U.S. median age jumped by one full year in this period, from 37.2 to 38.2 years. The median represents the midway point, meaning half the population is above that age and half the population is below it.

"This aging is driven in large part by baby boomers crossing over the 65-year-old mark," said Luke Rogers, the chief of the Population Estimates Branch at the Census Bureau, in a news release. The massive baby-boom generation includes those born between 1946 and 1964.

King County has lots of boomers, just like everywhere else, and we've seen a growing population of people age 60 and older here, too.

From 2010 to 2018, the county's fastest-growing age group was people 60- to 79-years-old. It increased by more than 100,000 people, for a growth rate of nearly 41%. This age group grew more than twice as fast as the population of the county as a whole.

Some of the increase could be from people in that age group moving into King County from some other place — the data doesn't specify — but probably not very much of it. Rather, most of the increase in the 60-79 age group is surely from already established county residents who aged past 60 at some point during the decade.

It's worth noting that the age group just below them -- the 40- to 59-year-olds -- barely grew in this period (a 4% growth rate). That's probably because of the large number of boomers aging out of this group as they hit 60.

So with a fast-growing population of older residents, how is King County bucking the national trend and getting younger?

Because unlike most places in America, we've also had a large infusion of youth this decade.

I'm not talking about a baby boom. I'm talking about millennials. Since 2010, the number of 25- to 39-year-olds in the county increased by more than 125,000, or 28%. A huge influx of millennials basically offset the county's growing population of older people, bringing down the median age.

You can see it for yourself. Just take a walk around South Lake Union at lunch hour. This neighborhood that barely existed 10 years ago is teeming with tens of thousands of young adults.

While the 60-plus age group grew primarily from people already living here getting older, that's not the case with these young adults. The Seattle area has been a magnet for millennials, with thousands moving here for jobs in the tech sector. Many others have come because they think this is a great place to live.

Census data shows that the median age of King County residents who moved here from outside Washington within the past year is 28.

There are now about 590,000 King County residents aged 25 to 39, accounting for more than one in four county residents (26%).

That's pretty rare. Among the about 1,000 U.S. counties with least 50,000 residents, there are only 28 (including King) where this age group makes up more than a quarter of the population. Many of the others are in counties that are demographically similar to Seattle — San Francisco, Washington, D.C., Denver, New York, Multnomah (Portland).

Elsewhere in Washington, the median age declined in three other Washington counties between 2010 and 2018: Adams, Kitsap and Garfield.

The state's oldest county is Jefferson, a popular area for retirees. Its median age is 58.9 years, an increase of almost 5 years since 2010. The state's youngest county is Whitman, home to Washington State University, with a median age of 25.3 years.

Interestingly, Jefferson is the nation's third-oldest county, while Whitman is the third-youngest, among those counties with at least 20,000 residents.

Overall, the state's median age increased from 37.3 to 37.7 years. Maine is the nation's oldest state (44.9 years) and Utah is the youngest (31.0 years).

Forty-nine states were older in 2018 than they were in 2010. The one exception, with a decline of nearly two years in its median age: North Dakota.

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