How to explain the current migration of the population from developing countries to developed countries?

How to explain the current migration of the population from developing countries to developed countries? Why is there a high proportion of old people and a small proportion of children in Western Europe? Why is there a high birth rate in developing countries?

In Western European countries, infant mortality is low and life expectancy is high because the standard of living is very high. There is a high percentage of people over 65. The nog ‘N factor is that European women tend to get married late and don’t want to have children.