Along with different indices of quality of age data, the rate of age not stated is also one of the important measures of the quality of age reporting in the census. The present study tries to explore the levels, trends and differentials in age not stated in India and the major states over the period 1971-2011. Sex-wise comparison of age not stated over the five censuses shows a distinct pattern of higher rates for males apart from few exceptions. Age not stated is higher in the urban areas in comparison with rural areas. It may be noted that except in 1991, in all other censuses, rates of age not stated are higher for illiterates compared with literates in all the major states. Moreover, omission of the population in the last two censuses is found higher than previous censuses. The same pattern is observed for age not stated too. Hence, it may be a problem of coverage with proxy reporting of the population where all characteristics can be probed but age is difficult to be reported by other persons. There is also the possibility of household members not reporting the age of young children. Moreover, this can be a problem of processing the information.