In China they link the ID to a phone number (via mobile carriers) and the online services require you to authenticate using the phone (SMS etc.) Unless the kids are able to secretly access the parent's phone there's no low-effort way to work around the system.

I don't know about Korea but if memorizing an ID number works, then that's just a badly designed system.

I'm not sure what your argument is really, unless you're saying there's technically and absolutely no feasible way to securely verify the age of a person before allowing them to access an online service (even if you allow the government to be authoritarian)

The point is, where there’s a will, there’s a way.

when i signed up for mobile service or for internet service in china (i don't remember the specifics), i was given half a dozen sim cards for use in my family. so they were all tied to my or my wife's name, but used by anyone who needed one. i believe the in-laws got at least one or two, and my kids would have gotten one, had they been old enough to have their own phone. i don't know if there was any rule that would restrict who we give those cards.

the actual users of each simcard did not have to identify themselves. so at least then it wasn't about age controls, but it obviously would allow tracing the owner eventually.