15-20 to start feeling it in the most developed nations, end of the century worldwide. Immigration can stall the former but current policies are not promising. Germany will sing songs of praise in Merkel’s name.
I am always amazed at news reports from Japan about how the sky is falling because their population is shrinking. For some reason no one ever says "on the plus side, Japan is one of the most densely populated places on the planet, a slow decrease in population will help improve quality of life measures and sustainability".