A drop down list of birth dates/years "works" for most age restricted sites - I guess the logic is that if a user is lying about their age, it's not the sites problem.

Article states that sites must demonstrate they are taking reasonable measures to enforce this though - a lot will come down as to how courts interpret that. If they go to the extremes of the KYC laws in australia I imagine a significant fraction of adults will not want to verify their age.