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 Guyana.
It is prohibited to sell or serve alcohol to a person under 18 years of age. The law provides an exception for minors aged 16 and 17, if they consume a glass of beer, wine or cider with a meal in a restaurant. Otherwise attempting to buy alcohol or buying alcohol as a minor is an offence.
You can find more legal details and links here:
Wikipedia Legal Drinking Page