>> Aussie law did not affect her ability to access socials while in Australia

I think that's expected.

>> Apparently the age checking there only applies to newly created accounts

Social media companies had to try and identify existing accounts owned by < 16 year olds and start removing them at the start of this year. I'd guess that process is slow and they don't do it unless they're certain. But if they stop new accounts effectively then within a few years the ban would be pretty effective.