When I was an early teen I had access to the internet but my activities weren't entirely unsupervised (and I doubt yours were either). Since it was a new technology there was a lot of discussion around how best to talk to children and make sure they felt safe reporting threats or harms to parents.

A smart phone is too disconnected of a device when compared to the desktops we all grew up on. No one is talking about fully banning <18s from the internet (at least no one serious) - it's a discussion about making sure that the way folks <18 use the internet is reasonably safe and that parents can make sure their children aren't being exposed to undue harm. That's quite difficult to do with a fully enabled smart phone.