At what age would you say it's appropriate to allow your child to have an iPhone?

When the leave the house without your direct supervision. Just make sure to control everything about the phone using one of the Nanny apps to prevent them from using apps you don't approve of or know about or taking pictures and sharing them, etc. Let them use apps your approve of only and of course make them call or text you when they are out of your supervision. With the Nanny app you should be able to find out where they are, or at least where their phone is. Most importantly talk to your child about the dangers of the Internet and what to do if they are uncomfortable with something and how to handle money, so when you give them a $300+ smartphone and the say drop it in the toilet, make them repay you with chores and whatever else until they get a new phone, etc.