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 Botswana.
It is prohibited for any person with a liquor license to sell or supply alcohol beverages to minors under the age of 18 years. Because the law only applies to licensees, it is not prohibited for minors to buy, consume, or possess alcohol beverages.
You can find more legal details and links here:
Wikipedia Legal Drinking Page