Ok, at which point an adult has taken responsibility for giving them access.

The older brother could also rent an R (or x) rated movie, buy cigarettes, lighters, dry ice, and give them to the kids. The point of the age check is to prevent kids from getting access without an adult in the loop, not to prevent an adult from providing kids access

This is a good point. We could extend it to computing devices: An adult gives a child access to a device, and now the adult is in the loop and takes responsibility. If said adult (parent, most often) want to automatically restrict certain activities/content on the device they can use the parental controls available. No panopticon required.

You can only keep the adult in the loop if you have a panopticon that traces back to said adult.

The system doesn’t have to be bulletproof. It just has to be better than the free for all it is today.

Better?..

Yes, there are good use cases for an anonymous age gate. So making one would be better than today's situation.