Several studies had incorrectly predicted that the aging of the "baby boom" cohort would cause prison populations to decline in the 1990's; this research attempted to determine the reason for this failure. Using a multiple time series design with State data over approximately 20 years, the analysis regressed prison variables on the proportion of the population in high- imprisonment-rate age groups (18-24, 25-34, and 35-44). A major finding is that age structure is, in fact, related to court commitments and prison population size. This is true for both males and females when studied separately. The failure of predictions that prison commitments and population would decline is caused by other factors that have stronger impacts on prison population size than age-structure trends in the general population. These trends will place downward pressures on the size of future prison populations, but these trends cannot be used to forecast prison populations, given the many factors that fuel an increase in the number of inmates. 5 tables, 1 figure, 3 notes, and 29 references