Why 12 ?
There is a hell of a lot of complexities to social media, connectivity , privacy , ect etc etc
Cultural and legal reasons more than scientific ones. 12 year olds can work in my country (California, US). With limited hours and required to attend full time schooling, unless the 12 or 13 year old is a high school graduate, then can work the same hours as an adult. In another state (my home state), you can legally drive under very specific circumstances at age 14.
The cultural angle is that in Western countries going back to at least the Middle Ages considered seven to be the "age of reason". And it was an age where a male child would interact with his father more, and in some cultures start choosing his own clothing to wear. Commonly known as Breeching in English speaking countries , as the age where a child approximately starts wearing breeches or trousers. (the buttons and fit were a bit complicated compared to modern clothing, I think a kindergartner today would struggle with them).
By tradition, Jewish children become religiously responsible for their actions at age 12 or 13, and they aren't the only culture that sets a milestone around this age.
Likewise puberty has enough issues in its self, body dysmorphia , bodily changes, sexual desires, staring independence of thought and action
I'm mostly in agreement with your entire point. I can't think of too many instances where I'd trust a 12 year old to make decisions unsupervised. But I think letting them exercise autonomy and then receive a limited form of consequences is fine. The consequences that adults artificially impose on children are meant to be a substitute for real world and permanent consequences if we just Lord of the Flies our childhood.
Adults seem to have convinced themselves that letting their children play on a screen controlled by Zuckerberg, Bezos, etc. for several hours a week is safe. When I'm more in the camp that it is harmful at any level of exposure, perhaps with the exception of direct adult supervision. (i.e. you look, I click, and I explain what a bunch of B.S. it is)