But I don't want to use the cookies, I used youtube-dl exactly to watch age-restricted videos without signing-in. I don't want YouTube to track which videos I watch and make history-based suggestions so I never sign-in and reset cookies often. I occasionally stumble upon age-restricted videos and feel curious what's there but I don't want any of them to influence what does YouTube believe my interests are so I just use youtube-dl INSTEAD of signing-in.
And by the way, I always mostly used random birth dates whenever registering anywhere because I consider that personal information and don't want to share it. I don't even know what birth date did I set when I got my GMail back in the days it required invitations.
I don't feel like watching an age-restricted video ever again if this actually requires identification and leaves no workaround.
UPDATE: I'm actually surprised this gets down-voted.