People who say that we managed before to walk home without phones etc seem to forget that public phone boxes rarely exist now, many places don't have landlines, there is much less of a culture of passers-by or strangers feeling like they can help a child, there tends to be an assumption that people do have access to internet, so far fewer easily accessible train/bus timetables on platforms/stops, fewer announcements or signs communicating delays, fewer people at information desks/ticket offices to help, fewer clocks on display, fewer places to buy tickets or whatever with actual money etc.
I don't particularly think children need phones before year 6 or 7, but it's totally up to families to decide what works for them.
By Year 7, a lot does tend to happen by social media, and without it, children do end up missing out. Unless you can convince everyone to stop using it, that will happen. No, social media use isn't great and can lead to a lot of problems, but I think phones/app/internet is here to stay, so better to teach children how to manage it well - I don't think the genie is going to go back in the bottle. I would try to avoid it until Year 7 or 8 (or later, if the child isn't desperate for it) but I'd understand the desire to fit in socially. I think encouraging parents to consider delaying social media is one thing, but trying to get them to come to a collective agreement is interfering too much.