Earlier quoted context omitted.
One example: in Japan cell phone operators provide a mechanism by which you can OAuth into their system, and then they will tell you whether the user is over 18 or not. This lets Line age-restrict "become friends with random people online" functionality.
That doesn't scale well. How does it expand to PCs, SmartTVs, tablets, games consoles, etc?
People might talk about burner phones, but many localities never really had burner phones in the first place (Japan doesn't give a phone number on travel SIMs, there's "no way" to have a phone number without getting your ID checked somewhere), and other localities are tightening the grip on this.
It's not a universally nice and open solution. But it's probably one most markets will bear