Comment by jeroenhd

Not necessarily. You need some source of truth (i.e. government ID) to sign digital tokens representing attributes like "18+". Those tokens are uploaded to those websites.

The risk becomes "kids loading their parents' ID into their phones" but with decent digital ID that shouldn't be a problem.

Yivi already solves this problem. It's being used as a basis for an implementation of a European digital ID of sorts, though I'm still sceptical of the European side of things.

The app works on any device because the device doesn't do anything special. All it does is POST some signed token if the user clicks "approve".

I suppose this can be a problem in the US where people hate the idea of digital government ID for some reason, but that's a political problem, not a technical one. France already has a digital ID equivalent for use with government services, as do all other EU member states in their own way, so the source of these tokens is practically ready to go.