Interesting, why do you say "a decade ago"? Peaks are only identifiable in retrospect, but what would mark "peak baby" then, that was more peak that current events?
Ah, the "the "Everyone is Twelve now" theory of politics" doesn't mean literal young children. It refers to the mentality of the (often gerontocratic) leaders and their adult supporters. Infantilism, not infants.
I understand, of course, and was being mostly tongue in cheek. However, I do think there could be something to the idea that as the majority of the world has trended younger we notice a net decrease in perceived maturity across the population.
In addition, "worldwide" is misleading - the countries where the "Everyone is Twelve now" theory of politics is most evident are not the same countries where the population has recently trended younger.
It would make more sense (not much more, but more) to blame early life lead exposure in the generation currently at the top of the political pile. Or blame social media.