https://www.theverge.com/2018/1/10/16873752/pokemon-go-older-iphones-ios-11-niantic-end-support

Niantic is ending Pokémon Go support for Apple devices incapable of upgrading to iOS 11. Older iPhone models, like the iPhone 5 and iPhone 5C, will be affected by the incoming update, coming February 28th.

If you have a phone without iOS 11, you won’t be able to access your account, use your pokécoins, or use items in your bags. According to the developer, the reason for dropping support is simple: “This change is a result of improvements to Pokémon Go that push the application beyond the capabilities of the operating systems on such devices.”