For starters, it's the "O" in "AOSP":
https://en.wikipedia.org/wiki/Android_(operating_system)#Ope...
So, 2007-present, that's when.
Android is acknowledged as a Linux distribution. Linux, also known as GNU/Linux, incorporates significant GPL-licensed code. By contrast, Apple has used BSD derivatives for a codebase, and BSD licenses, while F/OSS, are not "viral" in the way the GPL is, so Apple is not required to redistribute source code, or submit their patches upstream, and they can make proprietary additions anywhere they like.