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 El Salvador.
It is strictly prohibited to sell or serve alcohol beverages to a minor under the age of 18 years. Violation of the law is punished with ¢ 25,000.00 and if convicted once more the license will be suspended for a period of six months. It is legal for minors to buy, attempt to buy and consume alcohol.
You can find more legal details and links here:
Wikipedia Legal Drinking Page