The latest version of Android is running on a tiny percentage of phones
Lollipop, which was released in 2014, is still used on 35.6% of Android phones. And Gingerbread, the operating system that came out in 2010, is still used by 2.2% of Android phones.
One of the problems for Android developers is the so-called "fragmentation" of the operating system as customers are split between different devices and updates.
This can make it hard for developers to ensure their apps are fully compatible across devices and operating systems - although Google takes steps to mitigate this.
The low number of people using Marshmallow is a big contrast to the amount of users who upgrade to the latest versions of Apple's iOS. Apple publicly displays the percentage of iPhone users who run the latest version of iOS - a whopping 84% of customers are running iOS 9.