So I've been wondering...

Why not just have ISPs block IP addresses? Have IP blocking default to on, but allow the customer (who is over 18) be able to opt out.

Then have the services split up under-18 and 18+ IP addresses?

Seems much better than having to have websites magically determine age. It puts most of the onus on ISPs, but would be fairly simple on their end, and little to no affect on adults or parents who decide to take responsibility.

(I guess this is more in context of Louisiana law, not necessarily UK, which has more of a encryption aspect as well.)