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 Tonga.
Selling, serving or supplying alcohol to anyone under 18 years is prohibited. It is also prohibited for minors to buy, attempt to buy or consume alcohol in any public place and any violation of this law is punishable by a $1,000 fine.
You can find more legal details and links here:
Wikipedia Legal Drinking Page