**Editor's Note: Click here for our ranking of March's hottest selling smartphones to see how Apple's iPhone fared against the competition.**
About all we know about the upcoming iPhone 6 – sometimes referred to as the iPhone Air – is what makes its way around the rumor mill.
Two devices? Faster processor? Better camera? Near-field communication? It might matter less about what new specs Apple delivers than it does the number of users who are ready for an upgrade.
At least one-third of iPhone owners on each of America’s four largest wireless networks are using a device that is three years old – or older, according to a new survey from Business Insider. A whopping 56 percent of iPhone owners on Verizon are using an iPhone 4 o4 4S (the 2010 and 2011 models, respectively). That number is 47 percent at Sprint. Only 42 percent of the iPhones on AT&T’s network are that old, but another 10 percent of AT&T customers with iPhones are using a 3G S device (2009 version) or older. At T-Mobile, the number of folks carrying around a three- or four-year-old model is 35 percent, which is surprisingly high when you consider America’s fourth-largest carrier just started selling the iPhone last year.
Those numbers give us every reason to believe that Apple will see a huge upgrade cycle this fall.
Follow senior online managing editor @Craig_Galbraith on Twitter.