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 Costa Rica.
The law prohibits the sale of alcohol to a minor (anyone under the age of 18 years). Selling alcohol to a minor will be punished with a fine equivalent to ten basic salaries. Whoever permits minors to maintain in any establishment which main purpose is to sell alcohol beverages, will be sanctioned with a fine equivalent to five salaries. It is not prohibited for minors to buy, attempt to buy or consume alcohol.
You can find more legal details and links here:
Wikipedia Legal Drinking Page