15 is when I got mine, so I say that. But if they have extra curricular activities that would require them to need to call you to pick them up or something, then whenever that starts to become a major part of their life. I needed a phone at 13 because I had no way to call my mom after games or practices, but she still never got me one til I was 15, which I felt was unfair. I really hated having to ask people to borrow theirs or ask for rides.
It wouldn't have to be a smart phone or anything major, either. I don't think people should start off with those kinds of phones, except for certain reasons (waiting until a certain higher age, saving up for it themselves, etc.) I guess I'm somewhat old fashioned when it comes to children and technology, haha. But my step brother got an iPhone at 12, which I felt was unnecessary. He had an iPad by age 10 and two Xbox 360's, one for each house. That's been something that's shaped how I view these kinds of things, really.