Comment by somenameforme

This is where social media and other sites' endless datamining and profiling will come back to bite them. These sites already know the age range of users to a very high degree of certainty, and can continue to obtain such in an ongoing fashion. If an underage person is using these sites, it's likely going to be because the store clerk just nodded and winked, instead of because they were genuinely fooled by a borrowed or fraudulent ID. And in that case, the clerk is the one facing the penalties.

Put the burden of responsibility on the sites themselves and the number of people that will be able to successfully bypass such restrictions is going to be negligible and largely depend upon ongoing inorganic behavior or being an outlier in terms of behavior/interests.