It is often argued that population growth is not merely a question of numbers but that of the age composition of population. This is because

The age of 15 to 60 years is generally considered as working age or productive age. The people of this age is responsible to the addition in the net growth.

The population growth is not merely a question of numbers. It is a question of the age composition of population as the net addition in the wealth is when more people are in the productive age group.