Suppose you have 100 elderly people who are, on average, 1 year off from dying, and 1 infant who has a life expectancy of 100 years. You have to choose one of these groups to save, what is the most ethical choice?
1,000You or I do not have a right to decide, but the elders ought to be very readily willing to die for the baby. (The baby theoretically ought to be willing to die for the elders, but it is far more understandable for the baby to be unwilling to, and the baby would be outvoted, and the baby ought to respect the decision of the elders)
Ethics aside, if you're trying to maximize near term productivity, the people who survived to old age are probably going to do better than the average baby. But if you're maximizing population replacement, it should be the baby.