You can drink at any age in Belgium .
But you must be 16 to go to a pub without adult governance. You also must be 16 to buy alcohol in a shop. (but in most places it is no problem if you are not looking too young)
Of course you have also laws about child abuse (so a nip of a glass is no problem for a child but a glass is too much).
And there is a law about being drunk in public (and that is beside the law of drinking and driving).