"The purpose of the present paper is to review...recent...trends in population age structure in the world and its major regions, to assess the determinants of those trends, and to explore [issues] relating to recent and projected changes in the age structure of population and relationships of those changes to economic and social developments."  Comparisons are made between developed and developing countries concerning age structure, life expectancy, birth and mortality rates, and demographic aging. (SUMMARY IN ENG)