Older women's patterns of labor supply over the past forty years have differed markedly from those of younger women. Their labor force participation declined sharply during a period of rapid increase for younger women, and then increased significantly while younger women's plateaued and even declined. But there has been an apparent correspondence between the pattern of retirement among women aged 55-69, and the proportion of workers aged 25-34 working part-year and/or part-time. The latter was an effect of overcrowding among the baby boomers as they moved through the labor market. The former is hypothesized here to be a function of the increasing difficulty older women experienced in obtaining "bridge jobs" – part-year and/or part-time – between career and retirement. It has been demonstrated in earlier studies that older women – especially those in lower-wage jobs – often seek such bridge jobs before retirement. And in many cases these bridge jobs are not in the same industry or even occupation as the career job, leading one to suspect that in many cases there might be little transfer of skill or human capital. If this is the case, then the older workers would at least to some extent be in direct competition with younger workers for these jobs. Given difficulty in finding bridge jobs, a higher proportion of older workers might choose to enter retirement directly from career jobs, skipping the bridge jobs. A relative cohort size measure – the number of 25-34 year old women working part-year and/or part-time, relative to the number of older women, at the state level – has been shown here to be highly significant – both statistically and substantively – in explaining changes in older women's annual hours worked, labor force participation, and propensity to retire. In general terms, relative cohort size can be said to have generated between 15-30% of the observed changes in these variables, with the strongest effects being on the propensity to claim Social Security benefits. Somewhat stronger effects were found for older men, in a companion to this study.