The article introduces the tentative concept of deep aging – a label used for a situation when the overall ageing effects of the second demographic transition, due to fertility below replacement level, are topped by the excessive emigration of the fertile contingent (additional loss of active population). Deep ageing thus accelerates population decline. But the loss of fertile contingent may affect the total fertility rates and its apparent rise (TFR). Here, the tempo effect of fertility plays the decisive role. The article first assesses the demographic change in the European macroregions in the period after the fall of the Iron Curtain (1990–2020). It then analyses the changes in the area of former Yugoslavia to assess the extent of population change in the last intercensal period, 2011–2022. Building on the previous research of population loss and migration flows after the break-up of Yugoslavia, the overall population change in the region is being assessed. It is also shown that tempo distortion of fertility rates considerably affects the realistic level of fertility and that approximate completed fertility rates fell less dramatically. Since the migration data are less reliable, the analysis of intercensal change was applied to assess the migration losses across the ex-Yugoslav space. To assess the extent of deep ageing, the intercensal projection of fertile contingent was introduced. The stage of deep ageing is the most pronounced in Croatia and Serbia, especially at the younger fertile group, while the overall loss of fertile population is profound and will certainly affect the further future decrease in population. Similarly difficult demographic situation is in other post-Yugoslav countries, where only Slovenia has not yet slid into the stage of deep ageing.