Thank you very much for your insights, almost worthy of a blog post in its own right!
You make an important point: Favorable demographics are no guarantee for economic take-off and there remain huge variations and trajectories of fertility declines (or the lack thereof) across the regions of the world.
This leads me to another point. The global demographic dividend is naturally just an average that hides a lot of variation between and within countries. However, it also shows at that the same time that these opposing challenges (e.g. youth bulge versus aging) could also have global solutions, e.g. through smarter immigration policies.