How do they do it for movies ?
It is a matter of scale. There are around 800 movies released annually, that is about 1500 hours of film to rate. On youtube there are over 300 hours to rate every minute, or about 5 minutes you have exceeded the movie total. That means on youtube alone you have over 100,000 times the scale to deal with. Also note that MPAA (US) ratings are voluntary, paid for by the studios. In Canada, I believe several provinces pay into a government rating system. From what I understand Newfoundland has no rating system, and Saskatchewan uses the BC ratings.