This is the wrong approach. Social media is just as dangerous for adults as it is for kids.
We need to ban the current social media business models for everyone. That means: No advertising allowed; only show content from people or groups you actually follow; show content strictly in descending chronological order; do not allow sponsored content; charge people to have an account (Facebook probably makes around $3-$5/month/user, so that's a good benchmark.)
Also: Any sort of age verification as described still gives sites an oracle to let them determine the likely age of a user. If you insist on preventing people from seeing content based on age, the decision must be made purely on the client side. That is, the client will pull down the content as usual, but just not show it. That way, the Web site has no oracle with which to determine the age of the user.