Most developed countries are peaking in costs to young people right now. The people entering workforce now are getting a huge bad surprise, but the cost of supporting older people will start to decrease very soon.
So, if you are looking for some future phase shift, you are searching for the wrong thing.
Also, most of the developing countries will be in that situation in ~20 years. Most underdeveloped ones will get there in an extra decade or two.
the population bulge is at 50-60. with tfr as low as they are, we're looking at at multiple decades of a top-heavy pyramid. that's not disappearing anytime soon, it will take a lifetime.
So, if you are looking for some future phase shift, you are searching for the wrong thing.
Also, most of the developing countries will be in that situation in ~20 years. Most underdeveloped ones will get there in an extra decade or two.