Here is how I wish we would solve this problem :
Every adult citizen gets an electronic ID card. Said smartcard let's you authenticate on government websites (FIDO2). And maybe it could sign your vote for referendums if that's possible in your country. Every smartcard would be secured with a PIN code.
It also can create an anonymous "adult cryptographic token" to register on adult sites, gambling sites and social networks.
Parents could create a token for their childs but they take responsibility for any illegal use, for example in this case using social networks below 16 years old.
Edit : This seems to already be a project in the EU with the EUDI Wallet. It is supposed to start deployment before the end of 2026. Too bad the UK left the EU, this EUDI wallet has features to prove anonymously that you are an adult...
Now what really will happen is politics will say "think of the children" and use that opportunity for more surveillance of their population.
It's too bad technical problems are answered by politicians and not engineers. We are also very late in term of digital ID cards. Everybody should have for free the means to authenticate online and do so anonymously when needed. We live in a digital world, we need the tools to evolve in it.