iOS and Android Adoption Rates.

Android and iPhone compete for more than market share. Adoption of the latest software version by existing users is the current battlefield.
Appleā€™s newest operating system, iOS 10, is installed on 60 percent of iPhone devices according to the numbers Apple shared on its App Store developer support page.
Adoption seems steady at about 5%-10% a month. Here at Groove Development 70 percent is the magic number. We feel that it's safe to focus 100% on the latest version with some consideration for the previous version of iOS, depending on the client needs.

The adoption rates of iOS 10 has been slower than previous versions of iOS, but the installed base of all iOS releases seem to reach 90% within 6 months of release.

Apple's adoption rate is a joy for iPhone developers. Android on the other hand, not so much. The latest version of Android is 7.0 (aka Nougat), released in November 2016. Currently only installed on 0.3 percent of Android devices. Most users are running 5.x or 6.x and all new devices are shipped running 6.x(Marshmallow). It took a while for Andoroid to get version 4.x (Jelly Bean & KitKat) to 70 percent, version 5.x was faster. The pace of Android adoption has been incressing over the last few versions, as Android developers we are happy with that.

To Androids defense, it takes a lot longer to get updates out to users because the updates need to go through carriers like Verizon, T-Mobile, Sprint, etc. Each carrier has modified the user interface and in some cases added their own extras, so it takes time to release for specific phones.

Apple's iPhone iOS8 adoption rates as reported in November 2016.
iOS 10, 60% of devices are using.
iOS 9, 32% of devices are using.
All previous versions of iOS are 8%.

Source: Apple

Google's Android adoption rates as reported in November 2016.
Android 7 has 0.3% of the market.
Android 6 has 24.0% of the market.
Android 5 has 34.1% of the market.
Android 4.4 has 25.2% of the market.
Android 4.1-4.3 has 13.7%

Source: Google

Froyo = 2.2
Gingerbread = 2.3.3 - 2.3.7
Ice Cream Sandwhich = 4.0.3 - 4.0.4
Jelly Bean = 4.1 - 4.3
KitKat = 4.4
Lollipop = 5
Marshmallow = 6
Nougat = 7