Nigeria is the most populous country in Africa and the eighth most populous country in the world, yet there is a dearth of published research about its demography. As Nigeria enters a period of potentially rapid economic growth due to the increase in the working-age population, it is critical to understand the demographic trends in the country. This paper examines the age and sex composition of Nigeria as it relates to various population characteristics using the two most recent Demographic and Health Surveys for Nigeria (2003 and 2008), as well as some data from the 2006 Census. It also highlights Nigeria’s demographic composition and trends using United Nations population projection data, and its implications for the country reaping the demographic dividend. Finally, it draws some conclusions and makes some policy recommendations based on the findings.