No Stupid Questions

It is doable, i think. Consider:

You go to your local library. They verify you're above the age limit (like they do at supermarkets when you try to buy alcohol: either look you in the face and recognize you're clearly old enough, or have to show them some kind of id, details vary.)

You pick a code (put your hand in a box and draw a piece of paper at random). Nobody knows what code you picked except you. If lots of people do this at the same time, it's impossible to accurately map codes to people's identity.

You scan the code (like QR code) with your social media app that you use, and it associates the code with your account. Now everybody knows you have a valid code associated to your account, but nobody knows your identity.

(The code could work something like a cryptographic signature, where you can show that you have a valid code without actually revealing the code, so others can't simply copy it. That's a technical detail that you need to leave to the programmers to accurately understand.)

I'd say it's impossible. Minors will ALWAYS find a way around it, even if it involves government IDs. The actual trick is finding if a "are you 18?" box is enough or not.

There are tonnes of ways but honestly, the easiest way is to do it at the ISP level. Have an internet connection you don't want used for adult material? Have an opt in service at the ISP to block XXX rated sites and maybe social media. If you are old enough to pay for your own internet you should not be required to jump through hoops to access what you want, but kids should not be thrown onto the internet without guardrails. Some kids will get around it but it would be an active choice, so most kids would not. And to be clear, this would be done at the ISP level where you already have verification of age built in to billing, so no additional privacy concern. Honestly, the fact that this is not the solution is what tells me all of this filtering is not about protecting kids, it is about centralisation and control along with pork barrelling for age verification companies.

We could go by width of feet... You take a foot picture, it goes to another user temporarily while they sign in. If they think that's an adult then you're in. Then they take a foot photo and the next random user sees their image and they have to judge if the image is an adult. But the image could also be a fake. If they identify a false positive they have to wait 5 minutes. And so on. If others need to login they could judge the same image already identified as adult. If they think its not an adult then that user's logins is set to wait 5 minutes while two other users are snown the image. If those two users think its an adult then the user who said it was not an adult get to wait for 15 minutes. If however those two guys agree with it not being adult then that user gets permanently banned.

There are no registration or registry with individuals' information if guardians use parental controls and adult sites and apps identify themselves as adult for those controls, check what their kids are doing online, and talk with them about dangerous people or content they might see to teach them how to stay safe.