Why does the government need to know what pages you visit? It could just encode a string representing the date and 'above 18' and sign it with its private key. PH then just needs to verify it using the public key.

Even better solution then.

>>Why does the government need to know what pages you visit?

In the UK the government already knows, but that's beside the point. It's another bad law that should have been fought against much more than it was.