The most popular flavor of Android is Gingerbread (2.3.x) which was released back in February of 2011. This snapshot is taken from the Android developers site and shows the distribution percentages of Android operating systems in the wild.
In comparison, if you look at iOS distribution, you see that iOS devices running older operating systems are much smaller (source: Chitika).
So, as an app developer working with Android and iOS, there are clearly native challenges. On iOS, if you develop for iOS 6, you’ll be hitting close to 85% of the market, where as if you develop on Android, you’ll have to make sure your app is backwards compatible with 2.3.x and 4.0.x and 4.1.x.