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 Papua New Guinea.
It is prohibited for a minor to purchase, consume or attempt to purchase alcohol beverages. Any licence holder has to ensure that the customer who he sells or serves alcohol to, is at least 18 years of age. Violation of this law is punishable by a fine. Liquor Licensing Act (1963) Art 102-103
You can find more legal details and links here:
Wikipedia Legal Drinking Page