Hmmm. Zeihan likes to play it a little fast and loose with the details, but is entertaining in a doomey kind of way.
On a US basis the Millennials are bigger than the Boomers. And are already powering the US economy at this point. The 'lull' between the two (my small generation, 'X') came in the 2000-2015 doldrums, where the US turned to 'financialization' to maintain some semblance of economic growth. That didn't really work out too well.
Population is still growing semi-rapidly in Latin America, and rather rapidly in Africa, although current UN peak population projections are are much lower than they were 10 years ago. Progress!
On a global basis, those two regions are still driving global population higher rapidly, probably until well after 2050.
Given that decarbonization really needs to be well underway by 2050, the slow future decline in global population will
come too late to have a significant impact.
More importantly, global populations will get richer and demand more energy
services, leading to growth of end-user energy (not primary inputs) by
2-3X current global levels.
The critical issue is if that growth will come from low carbon and sustainable sources, which will make it manageable for the climate, or if they will come from fossils, which would lead to climate breaking emissions (like 2-3X current in a few decades).
So, decarbonization is actually a global fairness issue. There are enough fossils in the ground for the (current) global poor to have a middle class lifestyle (including, um, meat and dairy consumption), but not enough atmosphere to stash all the emissions. So if we want a middle-class world with a livable climate, renewables are the Way.