Parents are responsible for their children. If a parent doesn't feed their kid, they go to jail. If a parent harms or allows harm through negligence to children, the parent is the one who suffers the consequences and has the child taken away.
If a parent is giving a child a phone and allowing them to use a harmful product, the parent is at fault and should suffer the consequences. Not the rest of us. I don't know why I should have my access to anything restricted because of bad parents. Parents choose to be parents and have and/or keep children and that is their business. Bad parents should suffer consequences and one of those can be no longer being allowed to be a parent.
It's one thing if a provider is specifically trying to get children on its platform - and if a company advertises its services in public places, it's again on the parent to be in control there. Social media companies aren't holding a gun to children's heads trying to get them to join. Kids wanting to do stuff because other kids think it is cool has always existed and that happens when children are not supervised or disciplined. Kids not doing what they are supposed to be doing of their own choice is a parental failure.
Someone under 18 shouldn't be able to purchase a cell phone, and if a parent wants to get them a cell phone, then the parent should accept responsibility for everything on that phone.
The addiction argument is tired. Anything pleasurable can be addictive. If you want people addicted to less things, design society where everyday life is less boring (getting rid of 2 hour commutes and having more parks would be a good start).