Cheap phones for middle school. That's about the point where it becomes problematic not to have them.
Teachers start using text alerts, they are in activities where they need to be picked up, and of course its all communications with friends, etc. By middle school, not having a phone is a really significant functional impediment, and I'm not talking about just being a social outcast.
And by that age, I WANTED a constant line of communication to my kids. I mean, how often is a first grader really out of your sight or teacher supervision? I mean, you don't drop a 7 year old off at the movies by themselves.
But by middle school, they're starting to do more things on their own and in groups, and I want to be able to reach them. They need some independence to learn how to function. And I don't know how my folks dealt with situations like planning to pick me up at all mall entrance, and us instead going to the other entrance, and being totally out of contact for an hour until we figured it out. I don't need an hour of my time wasted, especially if that hour would be spent wondering if my kid was in some perv's trunk. I wanted them to be able to call me if they were at a friends house or something and felt like they wanted to leave.
However, before that point, I see no reason to provide one, and only bad consequences. They don't need a phone in elementary school. Hopefully by middle school their brains at least developed some ability to function offline.
That is not to say there aren't really compelling dangers there. Very strong studies have come out linking instagram to low self esteem and mental health issues for teen girls. It's really sad. There's no really simple way to counteract that, but you've got to do your best to build a really strong healthy foundation for your kids.
Maybe if all the things that suck about phones for kids went away as adults, I would be more inclined to just power through all the difficulty of not having a phone to get them out the other side. But none of that goes away as they get older, all the unhealthy image and reinforcement cycles continues to be a big problems for young adults and older. Eventually they're going to have to deal with it, and without the parental support system around.
It's kind of like driving. Is 16 to young to drive a car? Yeah, probably it would be better to have to be 25. But eventually, everyone's going to have to learn how to drive. It's still going to be dangerous then, and meanwhile you're driving a 19 year old around everywhere, or they're at the mercy of however irresponsible their friends are about driving. Some things you just have to bite the bullet hope for the best.