As home buyers across the nation find themselves frustrated by a tight market characterized by low inventory, some are placing the blame on baby boomers who refuse to budge.

According to Bloomberg, “Millennials are finding themselves out in the cold because building has slowed and longer-living baby boomers are staying put.” This has set up “a simmering conflict between the two biggest generations in U.S. history.”

Indeed, 53% of U.S. owner-occupied houses are owned by those 55 and older. This is the largest share since the government began collecting data in 1900, up from 43% ten years ago, according to the real estate website Trulia.

By contrast, those ages 18-34 own only 11%, nearly half the number baby boomers held at the same age.

“The system is gridlocked,” Dowell Myers, a professor of urban planning and demography at the University of Southern California, told Bloomberg. “The seniors aren’t turning homes as fast as they used to, so there are few existing homes coming online.”

Bloomberg cites public policy as part of the problem. Property-tax exemptions for longtime residents incentivize older people to stay put. Meanwhile, zoning rules make it “harder to build affordable apartments attractive to senior citizens.”

It’s also a problem caught in a vicious circle: the more seniors that stay put, the fewer homes there are on the market; and the fewer homes on the market, the less choices seniors have when thinking of moving; thus, the more they stay put.

It’s a problem that continues to confound industry observers. Millennials’ needs aside, for seniors to move, “they’ll have to have a landing place,” notes Myers.