European fertility is down to 1.3 children per woman. That fact alone, if nothing else changes, means the population will be cut in half in two generations.
Fertility is not the only fact that determines population. People could have more babies. For Europe to maintain a stable population, the fertility rate would have to double right now. Think of every family you know. Their European equivalents would need to double their kids as quickly as possible. And the no-kid families - they need at least two right away. If the fertility rate changes by a tenth of a point in a decade that is big news. To double right away? Not gonna happen.
Perhaps many people could immigrate to Europe? Wattenberg, in Fewer, reports that the UN projected that Europe would need to increase its immigration rate 500% (and still up the birth rate a bit) to stay stable. Many European countries are already having riots about the immigrants they have. When the report came out with that helpful suggestion, the European Union told them never to write such a report again.
Things are no better in the Asian Tiger economies that have muscled into First World levels of development. Japan also has a 1.3 fertility rate, or worse, and they disdain immigrants even more than the Europeans do. South Korea is down to about 1.5. Singapore, 1.1. Macao, which is not a First World economy but is working on it, has the lowest Total Fertility Rate in the world: 1.0. One child per woman. For every two people now, only one will succeed them. That way lies social collapse.
The bright light in First World demography is the United States. With a fertility rate above 2 and immigration, legal and illegal, above 1 million per year, our population is likely to hold steady or even grow a bit by mid-century.