With something like a physical gift card.

Go to a store or kiosk, show them your ID card or driver's license, and they'll give you a card randomly chosen from the shelf with a code to activate the +18 version of any social network of your choice.

Each code could only be used once. People would have to buy more, at a symbolic cost, for each social network they wished to activate.

I would tend to be against this in the same way, but at least it would be something I could understand where the objective is actually what is being presented (protecting the children), albeit misguided, because to me it is clear that what is currently being promoted and proposed has nothing to do with age verification, but rather with mass surveillance, marketing and censorship.