Many things are harmful for everyone, yet we only ban them for children because banning them for everyone would create huge uproar. Make tobacco illegal and then we can talk about social media.
In any case, this is whataboutery. There are lots of harmful things that adults are allowed, but lots that are banned for everyone. There are lots of things that cannot be sold without meeting safety standards (cars, electricals), or that are only sold subject strict controls (prescription drugs, guns). We do not allow adults to have whatever they want.