This paper analyses the causes and impacts of Age-Structural Transitions over a century using empirical data from 1950 to 2000 and population projections from 2000 to 2050 (UN World Population Prospects, medium projection). Three phases of population momentum are observed: The first, one of “waves”, occurred because of disorders in the patterns of birth and death. The second was due to the success of family planning program. The third phase is one of waves that is occurring today and will unfold in future carrying with it incipient indications of long-term aging. A “window of opportunity” has been identified here to occur between 2020-2030, but only for a decade. At that stage the dependency ratio will fall below 45 per 100 at working ages. If the government were able to create enough employment opportunities carrying with them decent income, Indonesia could exploit a demographic bonus, because a large number at working ages with adequate incomes and high levels of saving (because the dependency burden is low) would induce increased investment in quality human resources.