Answer:
In most developed countries, couples prefer to have less children (even none at all). An example of a country with an aging population (and a low infant mortality rate): Japan. It has great medical infrastructures and the survival of infants at birth is really high (compared to Sierra Leone, where the mother risks dying while giving birth * a common occurrence there due to the lack of resources*).
But, even though Japan guarantees the safety of the child and the mother, not many kids are born (bc of the couple's choice-- professional career, cost of raising kids, etc).
In poorer countries, especially in farmer families, having many kids guarantees that the family will maximize its work in the fields (ex. imagine a farmer harvesting a crop of corn all by herself/himself....now, imagine that same farmer + his/her children...way more effective). In such places, the reverse effect happens: a huge birth rate (young population).
Explanation:
*Attached, you will find the population graph of an African country (which has a young population). A country with an aging one would be the opposite of what you're seeing (more ppl at the top and less at the bottom).
