You've heard it said that iPhones stay up-to-date far longer than Android phones. Now, we have a handy chart from Fidlee to demonstrate that point in living color:
Concerning the chart, green is good, very good. It means that the associated product could run the current operating system for its platform. Four years into its life, the iPhone 3GS was still up-to-date, and only slipped with the recent launch of iOS 7. You will notice that many of the most popular Android phones start out one year out of date. After only two years, they are all, almost completely out of date by two major revisions.
Click on the source link for more detailed information. Here's the upshot: iPhones, and iDevices in general, retain their value far longer, in part, because they remain more useful and up-to-date far longer. There is no product in the Android world that even comes close. It is just one more thing to consider when choosing a platform for the long-term.