It's going to be every website. There will be no place they will stop. You think a forum like this one where it's conceivably possible someone in a bad category could interact with someone under the age of 16, however unlikely, won't be regulated?

"But sir! The largest websites on the internet implement Government ID Age Check. Just federate with one of those, why are you complaining so much? Don't you want to protect the children or stop anti-Semitism or something?"