In order to view age-restricted videos, it’s a good idea to verify your age associated with your Google account. You can follow these steps to check your birthday:
Sign into Google+ on the desktop.
In the left nav bar, hover over Home and click Profile.
At the top, click the About tab.
Click Edit in the Basic Information section.
Verify your birthday and update if necessary.
Once you verify your age, sign out of the YouTube iOS app and sign back in. You should now be able to watch age-restricted videos. If your birthday is listed as over 18 years of age on your profile, please check back in a few days