Young U.S. adults are putting off marriage longer -- sometimes forever -- resulting in more single-parent families, a non-profit group says. A Population Reference Bureau report found for the first time, households made up of married couples with and without children dropped below 50 percent of all U.S. households. \"Married-couple families with children -- once the predominant household structure -- now are even outnumbered by one-person households,\" study co-author Linda A. Jacobsen, vice president of Domestic Programs at Population Reference Bureau, said in a statement. In 2010, 20 percent of all households included married couples with children -- down from a high of 44 percent in 1960 -- by contrast, people living alone now represent 27 percent of all households, the report said. By 2012, 46 percent of young adults ages 25-34 were married, down from 55 percent in 2000. The median age at first marriage continues to rise -- 28.1 for men and 26.5 for women in 2011. In 2010, 41 percent of all births were to unmarried parents, up from 33 percent in 2000. The steepest increases in non-marital births were among women in their 20s. Women are having fewer children -- the U.S. average is 1.9 children per woman. However, from 1980 to 2010, the women ages 40-44 who were childless went from 10 percent to 19 percent. e t