For many Americans, the phrase "young single mother" conjures up a picture of a teenage high-school dropout. But that image is out of date. Teen pregnancy rates have been declining for two decades now. Today's typical unmarried mother is a high-school graduate in her early 20s who may very well be living with her child's father.

Despite her apparent advantages, however, she faces many of the same problems that we used to associate with her younger sisters. If 30 is the new 20, today's unmarried 20-somethings are the new teen moms. And the tragic consequences are much the same: children raised in homes that often put them at an enormous disadvantage from the very start of life.

Thanks in part to TV shows like "Girls" and predecessors like "Friends," we tend to think of today's 20-something years as a kind of postadolescent transitional period: Young adults move in and out of jobs and careers, hang out at cafes and bars with friends, test drive romantic partners and just try to figure themselves out. This pop-cultural depiction is accurate enough for the third or so of Americans who have a four-year college degree, but it's a long way from the reality of most 20-somethings. By the time they turn 30, about two-thirds of American women have had their first child, usually outside of marriage.

Indeed, 20-somethings are driving America's all-time high level of nonmarital childbearing, which is now at 41% of all births, according to vital-statistics data from the Centers for Disease Control and Prevention. Sixty percent of those births are to women in their 20s, while teens account for only one-fifth of nonmarital births. Between 1990 and 2008, the teen pregnancy rate has dropped by 42%, while the rate of nonmarital childbearing among 20-something women has risen by 27%.

The shift of unmarried parenthood from teens to 20-somethings is in part an unexpected consequence of delaying marriage. Over four decades, the age for tying the knot has risen steadily to a new high of nearly 27 for women and 29 for men, according to Census figures.