When you log in on the YouTube website using a browser, the site stores a cookie on your computer with all the information about your account. Trying to use the application without an account while watching a video with restricted age it will show you that error. The software already features a log-in box where you enter your credentials in order to bypass the limitation.
Please go to main window of the software then Options > Configuration > Integration> click on YouTube and enter your username and password. Please restart the application for the changes to take effect. Also, I have attached an image for easy identification of the setting I mentioned.