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, be careful as some establishments will accept only an in-state driver's license as proof of age.