What are the import problems faced by developed countries and their causes, harms and countermeasures?

Population problem: slow population growth;

The reasons for the problems are: high productivity, good social insurance and social welfare system, social economy and culture that affect the concept of fertility;

The harm caused by the problem: serious aging, insufficient labor force, insufficient national defense personnel, and excessive burden on young people.

Countermeasures: encourage fertility and accept immigrants.