If the effect on fitness of increase in mortality is zero, as is the case after the age when survival affects reproduction, then natural selection does not weed out the tendency for rates of mortality to accelerate with age. The idea that selection for reproduction in youth causes accumulating dysfunction in later adulthood is a commonly accepted explanation for aging.
If the impact on fitness of increased mortality is zero, as happens after the age when survival influences reproductive capacity, then natural selection does not eliminate the inclination for mortality rates to increase with age. The notion that early reproduction leads to declining health in later adulthood is a widely accepted explanation for aging.