With HTML5 more popular so it seems unnecessary for the YouTube app to launch every time you tap a YouTube video link on an iPhone, iPad, or iPod touch.
- Open Settings and tap on General
- Tap Restrictions – enable them if they aren’t enabled yet, setting a passcode in the process
- Flip YouTube to OFF and close out of settings
Now when you click YouTube links from Mail, Safari, or where ever else, and the YouTube app won’t open. Caveat Emptor is that it also hides the YouTube icon from the home screen.