Apple just released a study showing that 61% of devices are using iOS 9.
IT is important because the application Passbook or now Apple Wallet is built in and all iOS versions > 6 got this application pre-installed in your iPhone.
There is no need to download an App!
See below the iOS market shares from Apple Store statistics.
- iOS: 61%
- iOS 8: 30%
- Earlier: 9% (we estimate that iOS <6 represents now less than 1% of all iPhones)