As noted by Apple on its Developer site:
53% of devices introduced in the last four years are using iOS 12. As measured by the App Store on October 10, 2018 based on devices introduced since September 2014.
Juli Clover points out this is faster than iOS 11’s adoption rate:
iOS 12 adoption has outpaced iOS 11 adoption rates at the same time last year. In early October of 2017, iOS 11 was only installed on approximately 38.5 percent of devices. It took until early November for iOS 11 adoption to hit 50 percent, based on Apple’s official numbers.
iOS 12 adoption rates are in line with iOS 10 adoption rates in 2016. On October 11, 2016, iOS 10 was installed on 54 percent of active devices.