The radical improvement in living conditions experienced in Italy during the last century caused a reduction in male extra-mortality during the prereproductive years. As a consequence, a progressive increase in the sex ratio at the beginning of the reproductive age (15-19 years) occurred, so that in recent times the sex ratio in the young adult population has approached the almost constant value of 1.06 observed at birth. We calculated that the sex composition would be the same in newborns and in young adults in about one generation: obviously, we have to assume that the sex differentials in mortality and migration are constant over time. The 1:1 equilibrium between sexes, which maximizes reproductive success, occurred in the 15-19 age group at the beginning of the century and shifted to the 30-35 age group in the 1990s. We compared the 1993-1995 sex ratios in different age groups in European Union countries and observed that in Italy as well as in other Mediterranean countries the numerical equality between sexes is reached at 30-35 years of age, while in north-central Europe it is reached later, approximately at the end of reproductive life.