Alcohol Rules

Alcohol
First of all: the national minimum legal drinking age is still 21 years. It's also still the highest in the world and a controversial topic among the public, politicians, physicians and others.
In general every state has the right to establish own laws - these are called the MLDA laws. Every state including D.C. has them.
To be at age 21 or older does not mean that you have the right to drink everywhere you want to. In the first place the meaning is that one is allowed to purchase alcohol and i.e. to consume alcohol in pubs, restaurants etc. - but everybody should strictly be aware of additional regulations, i.e. don't drink on streets or in your car. Obey sign which indicate that it is not allowed to bring or consume alcohol, i.e. at beaches (at least at many of them).
When traveling check local laws: it might be that you're not able to buy alcohol on Sundays in some counties or cities. Some other places allow only to buy alcoholic beverages with less than 3.2 vol%. These are the so-called "3.2's".
The French Quarter in New Orleans seems to be dedicated to drinking folks - here you may also enjoy alcoholic beverages in public.
Brown-bagging
Sometimes a visitor will notice folks with little brown bags in public. This is a well-known way of 'hiding' and camouflage alcoholic beverages. Even when everybody 'knows', you're 'not really' drinking in public, at least nobody 'will know' what is inside the bag. Tricky - isn't it? But anyway, don't be misled and think that you can do this wherever you want to - it's not allowed, it' s just a way not to provoke and to earn some toleration.
ID check
Be prepared to be asked for a photo ID when buying or ordering alcohol - at least when you look young.
Liquor Licenses
Any establishment intending to sell alcohol needs this special license. It's not cheap for them to obtain it but alcohol beverages make up a noticeable portion of their earnings. most restaurants (except or Fast Food Chains) have this license.
If not you cannot take it for granted that you are allowed to 'bring your own' (BYO). BYO exists but is definitely not that common as it is in England, Australia, New Zealand and other countries.
Indian Reservations
It not allowed there to sell or drink alcohol. Possession might be prohibited!
Open Container Law
That means that you never should store opened bottles (of course with alcohol in it) in your car except of the trunk. When within reach of the passengers it would be a violation of law. Of course are many officers tolerant - in particular when there's no alcohol odor streaming out of the car... . Your car is of course not searched just when you run into a normal traffic control! But keep the law in mind.