A formula that obtains the number retiring in each period of a birth cohort's lifetime is derived from labor force participation rates and population data. It is demonstrated that trends in labor force participation of older men and in average retirement age need not move in the same direction. Applying the formula to the available data for American men, truncated working life tables between ages 52.5 and 72.5 are calculated for cohorts born between 1866 and 1909. The conditional mean retirement age is found to have been stable around age 65, while the variance of retirement age has decreased over time.