Do you realize that you are asking for mandatory proof of identity with government issued ID for every social platform?
Because that's the only way to make this happen.
Because that's the only way to make this happen.
With regards to social platforms, I think we need to consider the health of people under 18, just like we do in the real world. If we indeed decide as a democracy, that certain types of social media content are significantly harmful to children, then I think it is our responsibility as adults to impose certain things upon ourselves that may be uncomfortable.
I have gone to the beach and realized that I had left my wallet(and ID) in the car and was unable to purchase alcohol or be let into a bar. We as a democracy have decided that is a reasonable price to pay for protecting children from alcohol abuse.
If age verification were required to access online content, plenty of online services would be available as a third party to verify your age without requiring you to show every website and application your government ID. For example Apple might verify your age and relay that to a website. Or possibly your credit card company or some other new company which provided that service. PayPal for age verification.
I should qualify that I'm not sure we should impose these age restrictions on social platforms, I'm am instead commenting on the feasiblility of such an action.