[hollander]

In NL we have IDIN. A website can use it to verify age or address information. I don't think that any porn-websites are using it, but technically this should do the job.

IDIN is set up by all the Dutch banks. You check in via your own bank using an IDIN link. IDIN will do an age-check to verify that you're 18 or older. IDIN can verify age, name, address and contact info. You need a bank account, and the bank has checked your identity and age, so this is guaranteed to work.

Maybe you don't want that the bank knows that you visit a porn website, but that's another question.