Marissa from Google posted in the YouTube Help forums that if you want to beta test new versions of the YouTube app before they are released, you can.

They opened a method for iOS users to test new versions of the app using TestFlight, the iOS beta testing platform.

Marissa gave these instructions:

Sign up for TestFlight here. Note: there are a limited number of slots available, so you may be added to a waitlist.
Wait for a message from the iTunes Store that a new version of the app is available
Download TestFlight onto your iPhone, iPad or iPod Touch
Install the new YouTube version
Use YouTube just like you normally would! Be sure to report any issues through the app (Menu > Feedback)
Be sure to stay updated to the latest version. Users who do not stay updated may be removed from TestFlight at YouTube’s discretion (since there are a limited number of slots)

Have fun and be brave.

Forum discussion at YouTube Help.