A postwar baby boom is a temporary phenomenon where couples catch up on childbearing postponed during the emergency, mainly due to forced separations.
There is a direct relationship between economic development and reduction in birth rates. Since WWII birth rates have been falling essentially worldwide.
My understanding is most European countries have very generous benefits for parents, but those haven't boosted birth rates. Some countries have targeted incentive programs to try to boost birth rates, such as Romania, the Soviet Union and Nazi Germany, with mixed results, but with no long term impact.
Birth rates in Europe have been falling since the 19th Century. The trend is more recent in the Third World since development began there later, but those countries appear to be on the same curve. The UN predicts world wide population will peak about 2050, so the "greying" phenomenon will likely spread throughout the world during this century.
Immigration has put the US lower down the curve so the impact here will be delayed.
Your thought that cultural change could break this pattern is interesting. If, as some think, secularized, materialistic, Western Civilization is in decay and will be replaced by something new, perhaps such a change will occur. One example of cultural impact on birth rates is the Mormons of Utah. Even though Mormons are overwhelmingly European-American, they have not followed the Euro-American trend in birth rates, instead having birth rates comparable to many Third World countries.