That is…shocking

Likewise for cosmetic surgical modification of the penis by a doctor, or puncturing the earlobe by a non-medical-professional.

While there are exceptions, and in general exceptions seem pretty common, they still require businesses to officially get approval and it gives power to parents to enforce rules that would otherwise be hard to do so. Even with the exceptions for children to legally be allowed to drink, I would be surprised if that led to more kids drinking than alcohol obtained illegally, which means the question should be back on how well does the law work (obviously not perfectly, but there is a large gap between perfect and so poorly that it is useless purely from an efficiency perspective).

Is your position that no age limits should exist for anything?

This is already the reality for alcohol and plenty of other things. Maybe not everywhere. Reality check: parents giving unrestricted access to these things are usually perceived as irresponsible.

There's actually a really simple and elegant penalty - forfeiture of the device used to access the social media. With all seized devices to then be wiped and donated to low income school districts/families. This gets more complex when using something like a school computer, but I think it's a pretty nice solution outside of that. That's going to be a tremendous deterrent, yet also not particularly draconian.