Employment trends in the US were similar across age groups in the 1960s, 1970s, and 1980s: male employment rates declined or were flat at all ages and female employment rates increased or were flat at all ages. But employment trends diverged more recently, with employment rising at older ages and falling at younger ages, for both men and women. This paper seeks to explain this divergence. We estimate labor supply models for men and women, allowing differences in behavior across age groups. The results indicate that changes in the educational composition of the population and Social Security reforms can account for a modest proportion of the divergence. An additional factor for men was the increase in age at first marriage. However, much of the divergence remains unexplained.