It sounds like you will generally be okay, but I'll still contribute my $.02.
I don't think that there is really all that much that could happen in means of legal punishment, figuring the police would have to first, find the video, secondly identify that the person is underage, thirdly, that underage person is in fact truly drunk and not acting drunk, and then spend countless hours and thousands of the public's money tracing down the poster of the video and then all they way down to the drunk individual. So in those means, you are safe. And the only times that I have heard of legal punishments coming off of YouTube videos as evidence is cases of abuse and hazing, and usually those cases there are complaints to the police by the victim, and pointing to the video as proof.
Now in this case, I would be more worried of my parents finding the video, which there is a greater risk of them seeing it than the police. So I would be confronting your friend more on the worry of that.
And in the terms of an employer or college finding out, there is not a huge chance that they would find it if it were simply on YouTube, but if there were comments or anything that pointed to you, then I would be a little concerned.
So in this case (hypothetically of course), your greatest threat is probably your parents and then employer or school.
But then again, this past New-Year's Eve my friend posted pictures on FaceBook of her other friends drunk (although her and the people hosting the party were not). It almost led to the local high-school's athletic director losing his job after he punished the kids involved (the MIAA has a strict no-tolerance policy on its athletes being in the presence of alcohol, yet alone actually drinking it). Well, it was his house, his being away, his daughter hosting the party, and 15 kids showing up uninvited bringing an absurd amount of booze, followed by a mother of a kid at the party seeing the pictures online the next day. The biggest scandal the small town in MA has seen quite some time.
