I am fairly certain that it is possible to determine country by IP. I have seen this done before in several cases: some products with encryption are illegal to "export", and thus restrict the IPs that can access them. Also, I have seen the same for content providers who only have a license for a certain market (specifically I have seen a Japanese company restrict their content to "Japanese IPs").

You make a good point about proxying through different countries, but I don't think in you would be liable for that. Often these things work on some sort of a "reasonable effort" type clause, because its impossible to verify with complete certainty someones age programmatically.

Perhaps asking them trivia about music that was in style two decades ago...