Why can't I download iOS 7 on my iPhone 3Gs? I am unable to download iOS 7 on my iPhone 3Gs.  Why?  I have iOS 6.1.6 and if I want to download "Find My iPhone", it requires iOS 7.   I've tried to search on what else I can do, but cannot figure it out.  I also have about 10 GB of free memory out of 13 GB.
You cannot download it because iOS 7 is not supported on any iPhone model prior to the iPhone 4. The latest version of iOS that will run on the 3Gs is iOS 6.1.6 -- which you have.

Here's a fantastic compatibility chart for iOS 7:



Source: http://cdn.theapplelounge.com/wp-content/uploads/2013/06/iOS-7-Comparison-Chart-definitivo-1.jpg

Note the complete absence of the iPhone 3G and 3GS models.