Decentralized systems like the web already have a solution: lots of jurisdictions are making it illegal to provide adult content without age gating it. The point is for people to assume the same set of liabilities they would in person instead of the status quo where the web magically means you can do whatever. Then you just set up filters at home (or have ISPs offer following) to block the other jurisdictions. e.g. I lose nothing from simply blocking Russia altogether on my router.