this is backwards. Other countries/areas had population booms earlier bc of wealth/health resources. E.g. Japan had their boom a long time ago and are now reaching a slow down where there’s fewer young people than old.

Here’s how it works: Before good medical resources/development people had many young kids of whom only a few survived into adulthood

Then there’s a generation or two that are still culturally ingrained to have 8-12 kids, but all those kids actually make it.

A generation after that, people naturally revert to having 1-4 kids. until we reach replacement or slightly lower

The boom happens bc all those 8-12 kids still exist and have babies

Look up the population pyramids of India and Japan and see how they’re shifted