> Being adult is not a property of the user but of the jurisdiction that the user is in.

It's a function of both. And I'd argue it's really mainly a function of the person: Independently of jurisdiction, there's at least a rough global consensus what "being adult" means, and most jurisdictions set rather similar (many of them, identical) limits.

A four-year-old isn't an adult anywhere; a fourty-year-old is everywhere.