Other highlights:The number of couples living together without being married has more than doubled since the 1990s, from 2.9 million in 1996 to 7.8 million in 2012. In 2012, 40 percent of unmarried partners had children younger than 18. The prevalence of married households continued to decline, from more than two-thirds (71 percent) of all households in 1970 to under half (49 percent) in 2012. In 2012, 27 percent of households contained only one person, up from 17 percent in 1970. On average, American households contain 2.55 people. The median age at first marriage in 2012 was 28.6 for men and 26.6 for women. The share of all U.S. households headed by a white non-Hispanic adult fell to about two-thirds (69 percent) in 2012, down from three-quarters (75 percent) in 2000. The share of households headed by 55- to 64-year-olds rose over the last two decades, from 13 percent in 1990 to 19 percent in 2012. Meanwhile, the share headed by adults who were younger than 30 fell from 16 percent to 13 percent. The percentage of married couples with both the husband and wife in the labor force declined from 56 percent in 2000 to 52 percent in 2012. Stay-at-home parents have become more common since the 1990s. Among married-couple families with children younger than 15, the percentage with stay-at-home mothers increased from 20 percent to 24 percent between 1994 and 2012. The 2012 estimate of 24 percent is not significantly different from either 2011 or 2007, before the start of the recession. Almost all stay-at-home parents (96 percent) are mothers. However, between 1994 and 2012, the percentage of stay-at-home parents who were fathers increased from 1.6 percent to 3.6 percent and their numbers more than doubled from 76,000 to 189,000. The percentage of children in 2012 living with two parents, regardless of their marital status, differs by race and Hispanic origin. Eighty-five percent of single-race Asian children lived with two parents compared with 77 percent of single-race white non-Hispanic children, 66 percent of Hispanic children and 38 percent of single-race black children.