SSO/federated identity would be terrible for this. One of the biggest things people are criticising this idea for is the privacy implication. Somebody, in the current law, has to obtain private information on the user, such as footage of their face or their driver’s licence. Depending on how it’s implemented, the verifier might or might not be able to see details about your accounts on other sites or which sites you have accounts on. But in SSO, the verifier (or “identity provider”) must necessarily have that information. It would require whoever the identity provider is to have properly verified your age, and then every social media site (or, if this were a porn thing like in the US and UK, every porn site—in either case, this is the “service provider”) would redirect you via the identity provider, so the IdP knows which SPs you’re visiting.