Young-Old Threshold

Under modern mortality, the threshold is approximately 1.5 standard deviations below the adult modal age at death and about one standard deviation below life expectancy. The paper proves the threshold age exists and is unique under continuous mortality and mortality improvement at all ages. Mortality decline below the young-old threshold decreases lifespan inequality, while mortality decline above it increases lifespan inequality. Mortality improvements above the threshold age decrease Drewnowski’s index and increase lifespan variation. Mortality improvements below the threshold age increase Drewnowski’s index and reduce lifespan variation. Mortality changes near the threshold have little effect on the variance in age at death. The young-old threshold is always below the mean age at death after the index age. The threshold advances as life expectancy rises, modal age at death increases, and mortality becomes more compressed. Uniqueness follows because the threshold function is strictly decreasing. The threshold age determines whether mortality improvement increases or decreases lifespan equality.