What is the typical Android OS Upgrade policy? I noticed that our HTC Thunderbolts were upgraded from Android 2.3 to Android 4.0. That is very nice, I thought it would never upgrade.

Now that it did, I am wondering, why / what rules qualify an upgrade? Is it once every 18 months regardless? Is it diverse depending on which phone you have? Does it also depend on the carrier and/or brand? (are the HTC phones generally this way, or is it Verizon's choice)

Please clarify the Android OS update policy for me.

It would be extra good if you knew the policy on the iPhone OS, and how it compares.

Oh, and I already know that apps (YouTube, Play Store, Play Music, Gmail, Games) are not part of the OS, except for certain bundled apps. (Camera, Internet, Phone, etc.)
There isn't a specific Android OS Upgrade Policy. If it's a Nexus device and it can handle the latest version of Android it will get the latest version of Android from Google. Other than that it is up to manufacturers to update drivers and their specific customized version of Android for their own devices and then it is up to the carriers to approve those updates and release them to users' phones. 

Apple has only one phone basically and they release updates on what seems to be a yearly basis and update is done via their own service, not restricted by carriers.