According to the U.S. Census Bureau, married couples now represent 48 percent of all U.S. households. That's down from 52 percent in the last Census.
The flip in the 2010 Census happened in 32 states. In another seven states, less than 51 percent of households were helmed by married couples.
The reason, said Portland State University demographer Charles Rynerson, is twofold: The fast-growing older population is more likely to be divorced or widowed later in life, and 20-somethings are putting off their nuptials for longer stretches.
Experts say fears of not keeping a job, a widening labor market for women and a shift away from having kids at a young age are some of the reasons people in their 20s and early 30s are not joining the ranks of married people.
The median age for first marriages has climbed steadily since the 1960s, when men got married at about 23 years old, and women at 20. Now, men are waiting until they're 28 and women are holding off until 26.
Plus, attitudes on marriage are changing and more couples are deciding to live together without saying 'I do.' About 39 percent of Americans say marriage is becoming obsolete, according to a Pew Research Center study published in November, up from 28 percent in 1978.
The data supports that, as the Census Bureau reported last year that opposite-sex unmarried couples living together jumped 13 percent from 2009 to 7.5 million.
The Associated Press contributed to this report.