The difference between M and AO isn't the one year of age, it's about the message it's (trying to) send. It's like R and NC-17. If you're a young teenager with meddling parents, maybe if you're lucky your parents will look into an R rated movie you want to see, and let you see it. That's probably not going to happen with an NC-17 movie. It sends a stronger message about the bad content, which is what would happen with AO if more parents understood the ratings. Which they should.