Аннотация:We have investigated the age and mass distributions of individual stars in large-scale (300-700 pc) complexes of stars and gas in the Galaxy that have known age gradients. The stars we investigated are massive main sequence stars, low mass supergiants, Wolf-Rayet (WR) stars, and Cepheids. We have discovered a stratification of stars according to age: generations of stars are born sequentially behind density wave fronts propagating through H I superclouds.