This paper aims at assessing the robustness of demographic projections to different assumptions on mortality, fertility and migration. It builds on a small-scale simulation model for 23 OECD countries, which reproduces closely national projections under similar demographic assumptions. Up to 2020, projections are relatively robust to alternative hypothesis. However, uncertainty about future longevity gains and fertility rates account for a large range of results for dependency ratios by 2050. Eventually, a long lasting surge in fertility may not be enough to fully offset the impact on dependency ratio of increases in longevity in line with past trends (i.e., around two years every decade).