What shapes population age structures in the long run

We present and test a hypothesis that, to the best of our knowledge, is new in the demographic field: the age structure of any population in any period tends towards a specific shape, which can be identified in advance. This “attractor” is the age structure of the current stationary population, which we label RAS, or “reference age structure”. There is no mathematical demonstration for this tendency: however, we show that it exists in practice, measure the speed of the convergence (of the current on the reference age structure), and discuss the theoretical and practical utility of the notion.