I'm wondering whether we have any insight as to whether this is environment-driven (and particularly curious regarding places where we have a long cultural record of the effects of popular culture, so the US and Britain, for example)
You might try cross-posting this to r/AskAnthropology.
There is a lot of academic literature on this topic, much of it discussing the erroneous conclusions people have drawn from various Western-conducted studies on brains. Most of it is functional MRI research.
The upshot is that brain plasticity is a complex topic, and has little do with when “maturity” is accomplished or “personalities” are full blown. What little research there is outside the West indicates that whatever has been found in some Western studies is not a human universal.
In other words, no, it wouldn’t necessarily be the case for other time periods or other cultures. There’s variation within the Western individuals studied, and there’s virtually no research on why that variation might be occurring.
I don’t have my bibliography on this topic handy, and I’m on mobile so the following URL is a bit unwieldy: