If the answer above is None it means there is no specific mention in the legislation, but you still need to ask local law enforcement before drinking in Belgium.
Since 10 January 2010, it is prohibited to sell, serve, or offer any distilled alcohol beverages to those under the age of 18 or any alcohol beverage to those under 16. Previously, it was prohibited to sell alcohol drinks to under-16s, but accompanying adults could buy drinks for them.
You can find more legal details and links here:
Wikipedia Legal Drinking Page