The only thing I could find online regarding this was this stackexchange with the accepted answer claiming that there is no legislation, only a ruling from the Australian Classification Board against some media that they said “depicted [young persons] on the bordeline of 18 years old.” I.e. it was a discretionary ruling by a statutory regulator that thought someone looked a bit too much like a child.

There are some more links on that stackexchange but some are dead and I only skimmed a couple of them so I encourage people to read through themsleves since I was too lazy to sort the situation out for myself; I only found enough to satisfy my curiosity and thought I should leave this comment to add context to a bizarre claim (not blaiming prev commenter, my memory is horrible too and I also found some news articles that were definitely depicting the situation the way they remembered)

Part of the reason there’s not more evidence and it’s hard to prove a causal link is because it’s next to impossible to find enough participants for a control group because of how prevalent microplastics have become in our food and subsequently our bodies. You can’t exactly run peer review observational, experimental and double blind studies on only one half (more like one third, as a causal study would need to induce change from a->b) of the required test groups. t- and p-tests also are much less valuable if the sample size is too small.