The Strava app requires running iOS 10.0 or later on your iPhone. The iPhone 5 and above currently support iOS 10+.
Devices with iOS 9 as the last compatible OS (the Phone 4s and lower) can continue to use v9.0.0 of Strava if previously downloaded. No Strava app updates will be available.
It is not possible to download the Strava app to an iOS 9 device for the first time from the app store. An iOS 10 device is required to initially download Strava to your iTunes library, and then you will be able to install v9.0.0 onto your iOS 9 device.