That main ban is for purchasing alcoholic beverages. Adults in France can buy alcohol and serve it legally to their children. Same in Belgium, until 2010 it was even allowed to serve drinks to children in bars providing that adults where present and buying the drinks.

I don't get some countries or people that forbid alcohol until you are 18 or 21. I drank beer when I was 8 and got totally drunk at 14. Which was a good thing, because it was at home and, well, lesson learned. Starting with alcohol when going out at 18 or 21 sounds much more dangerous.