Privacy

You could probably make a pretty good run at it from GEO IP databases and ICAN blocks. It wouldn't be something the government is good at, but they could hire security contractors to put up somewhat effective barriers for people. Trying to enforce the rest of the world to do that for them is more complicated.

There are tons of ways to do age verification without leaking, if you had something opensource that digests something from an id and just signs a message in realtime that the user is of age, but there's a lot of room there for espionage. you REALLY need to be able to trust those apps.