The age you picked here makes your argument obviously right. But what age _should_ people get a phone and be able to use it unsupervised? I don’t think it’s reasonable that it be 21 or 18, even 16 feels quite puritanical.
I am someone who would probably not own a smart phone if it wasn't such a pain to not have one.
I can see with the kids in my extended family how even by 12, not having a smart phone is going to damage your social life. It was awkward enough for me at that age with rotary phones. I couldn't do that to my kid if I had one.
We have put ourselves in a really bad spot. Even if a parent wants to do what is best for the kid, I have no idea what that is. To pretend there is some simple and obvious answer here is completely divorced from reality.
Yes, that means not buying a smartphone and give it to your 3 year old.
I think, slowly, the world is starting to realize this.