Ummm, because we're white people. Duh.
Honestly the way I phrased that was probably a little misleading. The youngest probably won't be getting an iPhone, but I do have to get her a phone as well as add her to our plan, which will of course increase our bill. Both daughters starting asking for a cell phone in early elementary school and we took that stance that they could wait until middle school when it becomes a little more necessary. My oldest daughter didn't have a smart phone at all in 6th and 7th grade, just a cheap phone with the slide out keypad for texting that we got her as soon as her 5th grade school year was over.
My wife, oldest daughter and I all upgraded to iPhones last spring though (when my oldest was getting ready to go into 8th grade) and I have been thinking that we would may just go ahead and get the youngest one an iPhone as well. Odds are though, we'll probably get her a cheap smartphone that she can use for a couple of years. I want her to prove she can keep track of it and not lose it, for one.
I have been an Apple guy my whole life though and I have used a Mac my entire professional career, as well as bought Macs/iPods/iPads, etc, for my home. My wife and girls are already familiar with the platform so it is a pretty easy transition, albeit a more expensive option than some of the alternatives.