The U.S. home ownership rate is at the lowest level in 25 years and is widely expected to go even lower. That’s not just the result of younger Americans struggling to make ends meet to save for a down payment on a home. It is increasingly the result of middle-aged, higher income Americans choosing to rent.

Renter growth is now at the highest level in 30 years, and families or married couples ages 45–64 accounted for about twice the share of renter growth as households under age 35, according to a new study by the Joint Center for Housing Studies at Harvard University. In addition, households in the top half of the income distribution, although generally more likely to own, contributed 43 percent of the growth in renters.

“We do think we’re in the later stages of a rebalancing between owning and renting,” Fannie Mae chief economist Doug Duncan said in an interview Wednesday on CNBC.

Duncan pointed to demographics. Baby boomers are now moving out of their homeownership years, while Generation X, a smaller group by 6 million to 7 million, also has a growing preference to rent after being hit hard during the recession, losing income, credit and even their homes.

The homeownership rate is now 63.7 percent, according to the U.S. Census, down from the over 69 percent peak in 2004.

Because of that, rental apartment occupancy is now at an all-time high, and rents are rising at twice the pace of inflation. In turn, that is putting pressure on renters young and old, but not necessarily pushing them to homeownership. Higher rents mean it is more difficult to save for a down payment. More than half of U.S. residents report having had to make at least one sacrifice or tradeoff in the past three years to cover their rent or mortgage, and the highest segment of those sacrificing is renters (73 percent), according to a report by the MacArthur Foundation.

Majorities of Americans continue to believe that it is challenging to find affordable rental housing in their own communities (58 percent in 2014 and 2015), and housing to purchase (60 percent in 2015, 59 percent in 2014), and even more challenging for families at the median income level (65 percent), young adults (80 percent), or families at the poverty level (89 percent), according to the MacArthur Foundation.

Apartment construction is booming, but much of it is in urban centers, catering to wealthier renters.

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“It’s an older renter, looking to downsize that doesn’t want to own anymore,” said Douglas Firstenberg, principal of StonebridgeCarras, a real estate development and investment firm, standing outside one of its brand new rental buildings in downtown Bethesda, Maryland. Studios in the building start at $2,500 per month, and the most expensive unit is $6,000.

Rents are surging in the double digits for apartments and single-family rental homes. New apartment construction, now at the highest level since 1989, should ease the burden in coming years, adding supply to the demand, but it is not enough.

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“While affordability for moderate income renters is hitting some cities and regions harder than others, an acute shortage of affordable housing for lowest-income renters is being felt everywhere,” said Chris Herbert, managing director of Harvard’s Joint Center for Housing Studies. “Between the record level of rent burdens and the plunging homeownership rate, there is a pressing need to prioritize the nation’s housing challenges in policy debates over the coming year if the country is to make progress toward the national goal of secure, decent and affordable housing for all.”

Diana OlickCNBC Real Estate Reporter