Age structure and its dynamics are critical in understanding the impact of population growth on a country's growth prospects. Using state-level data from India, we show that the pace of demographic transition varies across states, and that these differences are likely to be exacerbated over the period 2011–26. We show that the so-called BIMARU states (Bihar, Madhya Pradesh, Rajasthan, and Uttar Pradesh) are likely to see a continuing increase in the share of the working-age population in total population. The BIMARU states are expected to contribute 58 per cent of the increase in India's working-age population. The BIMARU states have traditionally been the slow-growing states and have performed poorly on different accounts of social and physical infrastructure. The article argues that whether the demographic window of opportunity will be utilised and turned into a boon or be wasted and result in a bane will rest critically on the ability of the BIMARU states to exploit the bulge in the working-age population.