This paper reviews recent new trends in population structure in the world and its major regions in order to access the determinants of those trends and explore issues regarding the recent and projected changes in the age structure of population and the relationships of those changes to social and economic development.  In particular, the paper compares the change in age structure projected by the Population Division of the UN Secretariat in its most recent 3 series--namely, those completed in 1984, 1986, and 1988.  By and large, the most recent UN assessment projects that a larger proportion of the world population will be aged 60 and over in 2000 and 2025 than was previously estimated.  Those changes in projections can be observed for the world and for the more developed countries as a whole, and for the regions of Africa, Latin America, Northern America, East Asia, Europe, and Oceania.  While the recommendations of the International Conference on Population called attention to the importance of changes in population structure, this paper recommends urgent government action in planning social programs for the aged because of the greater eminence of population aging in many settings.  The case of Japan is used to illustrate the growing importance of increases in life expectancy as a determinant of age structure changes (in relation to fertility decline), a point that is reinforced through a cruder decomposition of UN estimates and projections for several European countries.