It hit Japan a while ago. It is hitting us too. Europe is facing the same problem.
We aren't having enough kids to replace the population, which is devastating for any economy.
(Third world nations are still growing in population, though. They buy our food, which is nice.)
That is why we need a well thought out immigration policy. Not one calculated to create societal tension and political problems to power the political elite, but one which let's in contributing members of society. For example, right now there are law abiding citizens of Latin America who want to come here who aren't willing to risk putting their lives in the hands of the Mexican cartels, and we won't let them in, but we should. We only reward those who pay the cartels. We are not doing anyone any favors.