A prompt will appear: "Download an older version of this app?" Tap to confirm. If You Haven't Downloaded YouTube Before
The official App Store will only let you download a compatible version if the app is already in your "Purchased" history. If you have ever downloaded YouTube on any Apple device using your current Apple ID, follow these steps: Open the on your iOS 12 device. Tap your Profile Icon (avatar) in the top right corner. Go to Purchased > My Purchases . Search for "YouTube" in this list. Tap the Cloud Icon next to it. youtube for ios 12.5 6 download