The following table shows the evolution of Android in the last 18 months, from March 2012 to August 2013.
It summarizes the data published monthly by Google on the various versions of Android available on smartphones. 18 months ago Jelly Beans did not exist (now has 3 versions) and Ice Cream Sandwich had just been released.
In March 2012 Ice Cream Sandwich was just released, its growth has been good until the beginning of this year, and then after the release of Jelly Bean it begins to decline. Ice Cream Sandwich is a version that has never achieved a good spread, it has never even reached 30%.
Jelly Bean has had instead a steady growth since its release and it is now about to reach the finish line to be installed on half of the devices.
All this waiting for Android 4.4 KitKat.
The last table shows the previous data aggregated by macro version number.