It's the same principle as the age of consent. It'll always be arbitrary, but that's still better than not having a limit at all.
I agree that something has to be done about kids on socials. But perhaps we can start with not punishing anyone by default and punishing offenders instead.
I don't see legally why a checkbox "not underage" isn't enough, as long as the rules are actually enforced and the people ticking that box while underage are punished appropriately. Instead of denying access to all unless proven above a certain age.