This article presents the demographic window in Mongolia and discusses the implications of different population policies for demographic development. Based on four different fertility scenarios, the development of future working age population (WAP), school-age population (SAP), and elderly population are examined, focusing on the economic and social challenges for Mongolia. It is argued that population policy could play an important role if policy-makers want to prolong the demographic window and the benefits of better demographic conditions to sustain economic and social development. This issue is of importance since the majority of developing countries are currently experiencing fertility decline which is opening their demographic window.