I know this is contentious, but one factor here that lines up with the timeline you've presented is: vaccines. Children are given far more of them than adults.

I know this is contentious, but one factor here that lines up with the timeline you've presented is: vaccines. Children are given far more of them than adults. People moving from developing countries to developed countries won't receive as many. But if they have kids in a developed country, their kids will get a ton.