Apple has stated that 93-percent of active iOS users are on iOS 6.

That’s fantastic. It’s great for us users because it means almost all of us are on the latest and greatest version. And it’s great for developers because it means they can have their apps support the latest APIs without fear of losing the vast majority of their potential customer base.

Based on some publicly available info from Apple, James Dempsey (Via Matthew Panzarino) put together this chart showing which iOS devices will be able to update to iOS 7 this fall. (You could preemptively add the new iPhone, iPod touch, iPad, and iPad mini that will all (presumably) be announced and ship this fall with iOS 7 pre-installed.)

Note that there are two devices on Dempsey’s chart that can currently run iOS 6 but won’t be able to run iOS 7: the iPhone 3GS and the 4th-generation iPod touch.

The question is: of the 93-percent of active iOS users who are on iOS 6, how many of them are using a 4th-generation iPod touch or iPhone 3GS, and thus won’t be able to update to iOS 7 this fall?

It’s certainly not a majority, but also likely non-trivial.

Which means it’s a tricky line for 3rd-party developers to walk while considering updating their apps for iOS 7. As a developer, you want to adopt the newest APIs and technologies and go “all in” with the latest iOS version. But you don’t want to abandon your customers who are on older versions of iOS. And the alternative of having 2 versions of the same app (one that is iOS 7-only, and one that is for iOS 6) can be a nightmare on many levels (logistics, marketing, customer support, etc.).

However, Apple is pushing iOS 7 as the biggest deal since the original iPhone OS. And I’ve talked to many developers who are getting the hint and strongly considering making brand-new versions of their apps which are “all in” on iOS 7, or else they’ve got an aggressive plan to turn their current app into one that requires iOS 7 as soon as they can.

Certainly the ease of use for all the current iOS 6 devices to update to iOS 7 via an over-the-air update will fuel adoption rates. And there will be a flood of new devices shipping this fall with iOS 7 pre-installed.

So the questions in my mind are about the short-term iOS 7 adoption rate be? And how long will it take iOS 7 to be running on more than 90-percent of active iOS devices?

I’ve got a guess, but I’m still working the math out.