It should be noted that these rules change over time. In Quebec, less than 50 years ago, it was still common to call your parents "vous". This was at a time when the catholic influence was felt very strongly, so the fifth commandment may have something to do with it.

Consider also that society as a whole is becoming less formal. Generally, the greater the age difference between two people, the more formal the interaction, so everybody older than you by X years becomes worthy of "vous" treatment. I find that this "X" number has been increasing steadily. Mostly, it's a good thing.