Age structure of any region is the consequence of birth rate, death rate, migration and the expectation of life at birth. This study examines the spatial and temporal changes in the age-sex structure of India’s population by studying in detail the shift in each age-cohort from 1881 to 2001. In addition, the paper examines the variations in the pattern of age-sex structure between the two regions of India with diverse demographic profiles – the four northern states (Bihar, Madhya Pradesh, Rajasthan and Uttar Pradesh) and the four southern states (Andhra Pradesh, Karnataka, Kerala, Tamil Nadu). This study is based on the hypotheses that a slow but gradual decline in fertility and a rise in the expectation of life at birth have changed the age-sex composition of India’s population; different age-cohorts have different demographic response to the impact of fertility and mortality change in the population; and change in the age-structure is more pronounced in the demographically developed states than demographically under developed states. The data are derived largely from Census. The major inference is that the change in the India’s age structure has been rather gradual till recently. The spatial pattern of the change in the age-sex structure is also at variance. Southern states have experienced faster change in the age-sex structure of the population than the northern states. Cohort-wise, change is more visible in the 0-14 and 60+ than the productive age-group.