On Long Island, along with the rest of New York State, you must be 21 years of age or older to purchase, drink in public or possess alcohol.
In addition, please note that in 2007, the Nassau County Executive signed into law Local Law No. 13, Nassau County's Social Host Law - A Local Law in relation to preventing the consumption of alcohol by minors at private homes. This makes it illegal for a person over the age of 18 to serve alcohol in their homes to persons under 21. For more information, see Nassau County's Social Host Law.