WRT to how it was done in Australia.

"Meta, which owns Facebook, Instagram and Threads, began closing teen accounts from 4 December last year. It said anyone mistakenly kicked off could use government ID or provide a video selfie to prove their age."

So the bulk is done as you say but they still need* an age verification system for when they over-stepped.

* need here is because of the way the laws were written AFAIK.