My old iPhone 4 doesn't show up on "Find my" even when logged in (under iOS 7.1.2). The problem seems to be that it must have the "Find My iPhone" app installed, but that app didn't come with this version of iOS. To get it, you need iOS 8, which doesn't work on this iPhone. Thanks, Apple! Anyway, download this old version of iTunes , which will let you install apps. Install "Find" on iTunes, then go back to the phone and try to install it from the App Store. This will let you install the 7.0 version of the app, which will then re-enable Find, and the world will be a better place. Hey wow first post here in 12 years! I should get a prize.
Here are the solutions I find on my own once customer support tells me they've exhausted all options.