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 United States.
Some states do not allow those under the legal drinking age to be present in liquor stores or in bars (usually, the difference between a bar and a restaurant is that food is served only in the latter). Contrary to popular belief, only a few states prohibit minors and young adults from consuming alcohol in private settings. The National Institute on Alcohol Abuse and Alcoholism maintains a database that details state-level regulations on consumption and sale age limits. See also: Minor in Possession and List of alcohol laws of the United States
You can find more legal details and links here:
Wikipedia Legal Drinking Page