What do you think the youngest age to own an iPhone should be?
whenever they can afford it themselves. I'm 18, got my first iPhone when i was 15, but I bought it myself (for $600!) and paid for the data plan myself (my parents paid for the $10 charge to get me on the data plan and family unlimited texting) with money from my job. I'm glad they didn't buy it for me, it made me more responsible because it was my money, so I was much more careful with it.
That being said, if your kid is interested in getting an iPhone, and will pay for it themselves, let them. They may learn some valuable skills from dealing with it, and, inevitably, computers because of it. In this modern age, technology skills are invaluable. Its a great learning experience if they're mature about it.