> but if a minor is caught using someone's credential, then the person whose credential they are using can be investigated, and, if necessary, charged with a crime roughly equivalent to providing alcohol to a minor. Without the possibility of real world enforcement, none of these identity solutions can possibly work.
They don't work even then.
Suppose you completely eliminate privacy on the internet and require every domestic site to collect the name and social security number of everyone who visits. Then a child uses an adult's ID, regardless of whether it's with or without their knowledge. Is the child going to inform on themselves? No. Is the adult, when they don't even know about it? No. Is the adult, when they provided it on purpose? No.
That constitutes the entire set of people who would typically know that the person using the device isn't the person on the ID.
On top of that, we can punch an even bigger hole in it. Search engines, among other things, index other sites. Google is obviously the biggest but there are many others -- Bing, Marginalia, Brave, Swisscows, Yandex, Perplexity, Baidu, etc. They're run by adults and most of their users are adults, who reasonably expect to be able to turn off "safe search" if they want to. So some adult at each search engine would have to provide their ID to the crawler so it can index things inappropriate for children and show them to adult users. It would therefore be a fairly unremarkable and recurring thing to see the same ID make a zillion gigatons of requests.
But then you can't use "why is this person downloading 100 things from 100 computers at once" as an indication of anything nefarious happening, and anyone can still set up a service hosted on a foreign server that will serve adult content to anyone without an ID by serving it out of a cache. (And in the case where you're invading everyone's privacy, that service would also be very popular with adults.)