Google’s Android celebrates fourth birthday on Sunday, September 23. Android version 1.0 ‘pushed out’ ion 23 September 2008 (Android origins go back years before that).
Android has turned to be huge success in those four years: Android had become the world’s leading smartphone platform with 500 million Android devices activated.
Platform versions page provides data about the relative number of active devices running a given version of the Android platform. This can help you understand the landscape of device distribution.