I don't see how such a "ban" could be effective. If you boot all minors from current social media platforms, they'll more than likely just migrate to an alternative solution - an alternative which will be less accountable and more opaque than the current ecosystem.

Possibly, but that alternative would have to fall outside the bounds of the relevant legislation for it to be something they could migrate to.

Young people have always congregated and that, in itself, isn't a bad thing (quite the opposite - they absolutely need to socialise). The issue we have is that current social media is a far too effective vector for bullying, exacerbating mental health issues and, of course, discovery and sharing of abusive material. Those are some of the problems we need to solve for.

As I say, I sense there may be a net benefit to a ban for younger people, but I could be mistaken - for example, there could simply be greater restrictions or regulation of social media use by young people that would make it safer/better - and would welcome evidence that proved me wrong.

I wouldn't say under-18. Start with under-14 or under-15.

Back in my own childhood I would be strictly against this idea. But the internet I grew up with is far different than the internet of today. It really is nearly impossible nowadays for clueless children to avoid getting sucked into some endless loop of being manipulated by people, social networks, or even the websites themselves.