From Wiki

Legal Issues

By and large, alcohol consumption rules are set by the states, and in some places, by the counties or cities. Closing times vary from city to city, and in some counties (primarily rural areas) they are still 'dry counties' in that you cannot buy alcohol in that area.

In many areas grocery stores can sell beer and wine, but not hard liquor, for that you will need to go to a liquor or 'package' store. In some states, the sale of liquor is controlled entirely by the state and you will need to go to an ABC (Alcohol and Beverage Control) store to buy it.

A few important rules:

1) You cannot drink alcohol in public. Taking a beer out of a pub onto the sidewalk is illegal in most states. Public consumption of alcohol, while tolerated at places like sporting events (the 'tailgate party'), drive in movie theatres and a few other situations, it is generally not allowed in the US. At most bars, you'll either have the cup taken from you at the door by security, or channelled into a 'beer garden' which is separated from the public sidewalk by a small fence. Walking down the street with a glass of beer is certain to get you a citation for 'public consumption'.

2) The drinking age is 21 (with LEGAL PROOF). You are 50, you have grey hair, you look like a geezer, technically, this is not enough to buy alcohol in many states. To get into many bars and buy alcohol, you often have to produce a government issued ID (drivers license, passport) that shows your picture and has your birth date. Some stores such as Hannafords will only accept a US drivers license or liquor ID (ID for people over 21 who do not hold a drivers license), so not even a military ID or US passport will be accepted, but this is usually the exception to the rule. It may seem silly, but many a bartender can tell you about how they have turned away grey-haired old ladies because they couldn't produce proof of their age. If you plan to go out drinking, take an ID and avoid any problems. Whether they 'card you' (ask for ID) varies, but better safe than sorry.

3) Driving while drunk is a big no-no. This happens thousands of times a night, and some bars miles from anywhere you have to just ask 'how the hell are you supposed to NOT drink and drive with a place like this." But getting stopped for a DUI test can really cause some problems with your insurance, losing your license for a minimum of six months in many states, and also creating a lot of problems for immigration and VWP. It's best to find a 'designated driver' (i.e. the buddy who gets free Cokes from the bar) and have them deal with you getting home.

Cultural Issues

Alcohol consumption in the UK is quite a bit higher than the US. About 1/3 of Americans don't consume alcohol for various reasons, and the number of heavy drinkers is less than 5%. "Binge Drinking"--drinking to the point of passing out (aka a normal night in most UK city centers) doesn't occur much outside of college and university towns in the US. Drinking during the day, i.e. a little something with lunch, is frowned upon by most businesses (though it occurs more often than anyone would admit).