Abstract
Developing countries will very likely enjoy an exceptional period where the proportion of adults of working age will reach a historical maximum for a certain time and where investment for future economic and social systems will be easier than ever. But, the more favourable this event, the shorter the golden age will be, and, very likely, the more rapid and acute the successive period that will bring ageing and related problems will be. This short paper aims to highlight the demographic background of the question. Exactly what does ‘demographic window’ mean? Different countries or groups of countries are in very different situations with regard to what could be expected theoretically. Such differences can largely explain the different economic and social situations of the countries.