I mean, we had similar discussions about having a computer and laptop back in the day, so really the modern dilemma is the phone/tablet aspect. Only real difference is portability, honestly.
I'm not a parent, so definitely take what I said with a grain of salt, but I think 8-9 sounds about the right age they should be allowed to get one? One, reduces the illusive nature to having one. Two, society is pushing it more and more and some countries had already pushed for this sooner than others. Three, and if I can be frank, I don't think it would hurt to start teaching and showing them how to use it early on and set boundaries of what's okay and what isn't.
You're saying until High School, but I feel like it'll get more difficult and I can't help but wonder if they'll get into serious issues without having been able to handle one a lot sooner, as well as understanding boundaries and stuff. Even at the end of my High School time back in 2007-08, there were plenty of incidents both on early smart phones and flip phones, about nudes and crap being spread around. I think this might be more important to kinda teach against this stuff than to have them end up dealing with it before it's too late, especially as the law hasn't caught up with this aspect of technology.
This is just my personal thoughts on the matter. Again, I'm not a parent, so take what I say with a grain of salt.