There are some things those of us born in the wake of World War II, the “Baby Boom,” take for granted. Like, for instance, that the sun revolves around us. And that we invented sex. And fun.

And motorcycles. Motorcycles, for sure.

So the recent news about Harley-Davidson’s hard times was more than a little unsettling.

Back in February, when Harley-Davidson’s CEO Matt Levatich provided Donald Trump with a photo-op outside the White House, posing with a slanting row of polished steel hogs, everything seemed great. The president may not have actually ridden a motorcycle but, being a Baby Boomer, he had certainly imagined himself in the saddle, at the head of a thundering column of fellow bikers, commanding the open road. Harley-Davidson represented the kind of American manufacturing he said he could bring back.

Now we come to find out that, according to Levatich, H-D is planning hourly employment reductions in its U.S. plants — and building a factory in Thailand. Details about job cuts, Levatich said, would be shared with workers at a later date.

It seems demand for motorcycles has been falling like hair from a Boomer’s scalp. Harley’s third quarter motorcycle shipments could be down by as much as 20 percent, and H-D stock has been downgraded. “Longer term,” investment analyst David Beckel told Bloomberg News, “things are getting worse, not better.”

What is going on here? It seems like only yesterday motorcycles were the ultimate alpha male accoutrement. Marlon Brando mounted a chopper in The Wild One; Steve McQueen made his getaway on one in The Great Escape; Peter Fonda and Dennis Hopper rode them on their stoned odyssey in Easy Rider.

If you are a Baby Boomer, motorcycles probably seem as American as roadside diners and bacon and eggs. Timeless.

In fact, motorcycles, like high taxes on the wealthiest earners and longterm job security, were a distinctively postwar phenomenon. As Hunter Thompson noted in his classic mid-60’s book, Hell’s Angels, there were less than 200,000 motorcycles registered in the entire United States after World War II. In the 1950s, sales doubled then tripled. When the Japanese began importing lightweight cycles in the early ‘60s, registrations went through the roof. Motorcycles, like rock and roll and coal-fired power plants, seemed here to stay.

Now a generational shift appears responsible for Harley-Davidson’s declining fortunes. Millennials, say stock analysts, don’t appear to be as interested in motorcycles as their parents and grandparents. This, on reflection, seems pretty obvious. We’re seeing more and more younger adults pedaling bicycles, instead of revving internal combustion engines. There are also growing demands for more extensive and efficient public transportation. While the lure of the open road remains, its appeal, for many, may be morphing into a variety of experience more virtual than actual.

All of this may be a hard pill for some Boomers to swallow. Like the Carrier workers who had their photo-op with Donald Trump, only to learn of lay-offs and, ultimately, how their Indianapolis jobs will be automated out of existence, the people making Harley-Davidson motorcycles are finding out America never stands still. It makes itself great (or not) one generation at a time.