See back of NCL brochure - as a general rule guests must be over 21 to buy alcohol but between 18-20 can purchase & drink wine and beer with parents' consent. I would double check this, as you are in US terrority where you have to be over 21 to drink.
As it says on NCL website: With the exception of Alaska and Hawaii itineraries, (Where you must be 21 years of age to consume or purchase alcohol of any kind) NCL permits guests between the ages of eighteen (18) and twenty (20) to purchase and personally consume wine and beer while on NCL ships with the consent of an accompanying parent or legal guardian on the same sailing once the vessel is outside the US state territorial waters (3 miles out). Complete details and special consent form are only available onboard at the Reception Desk. *If you want to gamble in our casinos, you must have a valid ID showing you are 18 years or older. If a guest under the age of 18 wins in the casino and can not provide proof of age, he or she will not be paid the winnings. There are no casinos on our ships in Hawaii.