Contrary to common sense, the population aging process is due to the decline in fertility rather than mortality. The aging process began around the end of the 19th century in a number of Western European countries, expanded to the rest of the so-called First World over the past century, and reached several Third World countries afterwards, including Brazil over the last decades. In the Brazilian case, a sharp and widespread fertility decline began by the end of the 1960s, and an accelerated population aging process can thus be expected. This process will necessarily be faster and with deeper structural changes, demographically speaking, than in First World countries, for two reasons: the fertility decline in Brazil was faster, and it took place in a population with a younger age structure.