The legal age for drinking alcoholic beverages is 21. Most establishments do take the law seriously because they can face personal fines and criminal charges if they don't. Hence, don't take it personally if they ask you for proof of age (a legal photo ID) if you're lucky enough to look under the age of 35. Take a photo ID out with you if you plan to drink or purchase alcohol, even if you look 35 or 40!

Other than the legal drinking age, different states have varying laws on the purchase of alcohol. In some states you can't purchase alcohol in a grocery store. In other states, you can't drink after a certain time of night, or you can't purchase alcohol if anyone in your party is underage (regardless of who is buying). Do some research on both local and state laws before you go.

For travelers from the UK: Older licenses have no photo on them, and as such, these are generally not acceptable as proof of age. You may want to consider getting a new EURO style license before your trip. Showing a passport is also acceptable as a form of ID.  If you are travelling to a college-town, such as Boston, some establishments will accept only an in-state driver's license as proof of age. 


"Open Container" Laws and Providing Alcohol to Minors:  There is a patchwork of laws regarding consumption of alcohol in public places (do a Google or Wikipedia search for details).  Generally, it is best not to walk down the street drinking from an open container of alcohol, even a beer, in public. Many public beaches prohibit alcohol as well. Also, note that there are strict 'open container' laws applicable to being in a private motor vehicle. Essentially it is forbidden to have an open container of alcohol in a vehicle on a public road, even for a passenger.  Because of the variations in different states it is best not to have an open container of alcohol in your car at any time.   Also be aware that the laws in the United States are very strict about providing alcohol to someone under the legal drinking age of 21, even your own child, so it is best to avoid doing this.