Over the past two decades, the share of individuals claiming Social Security at the Early Eligibility Age has dropped and the average retirement age has increased. At the same time, Social Security rules have changed substantially, employer-sponsored retirement plans have shifted from defined benefit (DB) to defined contribution (DC), health has improved, and mortality has decreased. In theory, all of these changes could lead to a trend toward later claiming. Disentangling the effect of any one change is difficult because they have been occurring simultaneously. This paper uses the Gustman and Steinmeier structural model of retirement timing to investigate which of these changes matter most by simulating their effects on the original cohort (1931–1941) of the Health and Retirement Study (HRS). The predicted behavior is then compared with the actual retirements of the Early Boomer cohort (1948–1953) to see how much of the later cohort's delayed claiming and retirement can be explained by these changes. The Early Boomer cohort was less likely to be fully retired than the HRS cohort at both age 62 (36.7% vs. 44.0%) and age 64 (49.5% vs. 53.9%). The model suggests that the shift from DB toward DC plans was the biggest contributor to these declines, followed by better health. Social Security rules and improvements in mortality played smaller roles.