World population will reach a maximum of 9 million and then slowly decline as birth rates have fallen massively everywhere. However, in some countries the birth rate has fallen so much that it will be a huge problem. In those countries young people will have almost no peers while growing up, and in the context of Democracy old people will have a majority. See the Kurzgesagt video for what live will be like in South Korea: https://youtu.be/Ufmu1WD2TSk