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 But that image is out of date.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...