Because its a half-truth. Women have lesser viable eggs as tbey age more because of toxicity in their systems, stress and bad ways of living than age itself. Obviously women in general have less fertility as they age, but women who are aware of their health prolong their fertility well. The Downs thing did not exist in ancient times. Women simply were healthy enough that their bodies ended the pregnancy as the genes of the baby were not compatible. Genetic defects are a mark of an unhealthy woman much more so than an old one.