It's destructive for humans to multiply like they have in developing countries indefinitely, but there is just as significant a threat to economies such as Japan and Italy, who have an inversed population pyramid... where there are many more elderly than children. It is best to keep a slight increase in population growth, such as 2.1-2.2 children per couple in order to ensure developed countries won't suffer the same fate such as what these states are facing.
It would be most important to establish and maintain the highest population/dependency ratio. This is a measure of the total number of a population compared to the number not working, such as children and elderly. If people are expected to live longer, then the age for retirement should change accordingly if they are going to be healthy enough to work another 10-15 years.