Based on what I see, the iToy is a right of birth. I'm stunned at how many toddlers I've seen who know how to "work" their parents' phones, or better yet, the number of parents who hand the phone over to the toddler to "keep him/her quiet" and entertained.
From my perspective, late elementary is probably the earliest appropriate age, and that's really only because of the number of businesses and people who don't have landlines anymore (and the absence of pay phones). Keb attends a weekly game tournament. Due to the fact that we don't want to sit there with him for 4-5 hours, he has a phone so that he can call us when he's done. Actually, it's more like we call him. The responsibility piece really isn't there yet. However, before we got him a phone, he used mine, and he never asked for one. It's just not the big thing in our community.
In my mind responsibility is the biggest phone factor. When your child reaches an age where he/she is completely responsible for this item that is or can be quite expensive, is responsible to your rules, and knows where the item is when you ask, then that's the right age.
When I taught 5th grade, a student desperately wanted a cell phone. His parents gave him one of their "dead" phones to carry for a month. If he had it every time they asked and didn't lose it, they would get him a phone. By the end of the year, he'd gone through three dummies. He kept saying it was because it wasn't real, but his parents felt that if he was serious, he would have kept track of his phone. I always liked that idea. It made sense and tested whether it was a want or a need. It also made sure they weren't going to keep paying for that phone.
In my mind, older is better than younger with phones. After all, if you don't know where your elementary school kid is, you have more problems than a phone can solve.