http://www.techradar.com/news/pokemon-go-wont-work-on-your-older-iphone-starting-next-month

It may finally be time to go to your nearest Pokémon Professor and update that Pokédex. 

Niantic has announced that it’ll be ending support for all Apple devices that aren’t able to upgrade to iOS 11. That means iPhone 5 and 5C — as well as iPads and iPad Minis purchased in 2012 or earlier — that can't meet the game’s growing computing needs.