Economists have landed on two primary reasons for why women working full-time earn less than their male co-workers. First, their research detects an element of discrimination. The second reason stems from motherhood, which can make it extremely difficult to simultaneously complete an education or get a firm footing in a career. But America is changing. Over the past half-century, the typical age at which women have their first baby has risen markedly, from 20 to 25. This societal shift toward later motherhood has, in turn, dramatically improved women’s financial prospects, concluded a study featured in a book about the financial impact of changing employment, family and health trends. University of Virginia economist Amalia Miller found that each one-year delay in…