Not to mention that banning under 18s means identifying every user: A government ID required to access the internet.
How about if I’m posting in a chat for an environmental protest group, or an anti-capitalist group, or against genocide, or in a queer support group in a country with laws against homosexuality? Should they have my Government ID attached as proof of age?
An age-gate is asking for an internet licence and a single unified monitored ID across the whole internet. It’s dystopian surveillance-state authoritarianism. It is not desirable, even if it were possible, which it isn’t given black markets in stolen ID etc.