If you are under 21 years old, you cannot legally drink alcohol in the USA, whether you live here or are on vacation in the country and come from countries where the limit is lower.
Even if at 18 years old you are no longer considered a minor and you can drive much earlier, at 16 years old; unless you are 21 you cannot buy any type of alcohol, nor be served alcohol in a restaurant or at a bar (indeed in most bars you have to show your passport or driving license to even enter).
I was so surprised to be asked for I.D. (identity card) even if I am well over 21 and I don’t look any younger than my age - unfortunately. But this is the law. If you look 35 years old or younger, they are supposed to ask you for I.D. before serving you or selling alcohol to you.
Many bars are strictly reserved for adults and minors cannot get in even if accompanied by their parents or legal tutors, others are adults-only after a certain time of the day.

Also if you serve alcohol to non-family minors, this is punishable in any case by Law, even if takes place in a private location.
Carrying open bottles of spirits is illegal in the States, just as it is illegal to carry packs of beers (12 or 6 bottles or cans) that are not complete inside your car!
Drinking in open, public places such as streets or parks is prohibited as well.
Laws are indeed very strict when it comes to drinking but I don’t think they help much curbing the drinking problem as many (especially young) Americans get wasted (completely drunk) during the weekends and many adults have a drinking addiction.

Unfortunately, consuming alcohol is seen as a rite of passage into adulthood and many youngsters, also below the age of 15, choose to drink. If a minor accompanies a friend to a liquor store or a supermarket he will be asked for I.D. as well and the alcohol won’t be sold to the friend just because they are together! But this is an easy hindrance to overcome: kids send a friend over the age of 21 alone to buy alcohol for everyone.
At times some friends of mine have been asked by a group of underage kids to go inside a liquor store to buy spirits for them. Of course they refused as it’s wrong and irresponsible other than being also punishable by Law.
A couple of funny events happened to me since I moved here.
One is that they asked me for I.D. in a cafe to order a cappuccino with a famous Irish liquor with very little alcohol content and I thought that wouldn’t be my drink of choice if my aim was to get drunk at my age.
The second and funniest episode I think is that I was asked I.D. to buy some chocolates with liquor (in a very small percentage) and, since I’m far from being under 21, I think it’s hilarious I would choose very expensive chocolates to get drunk.
That’s the bottom line unfortunately. Many young and less young Americans see alcohol as a way to get drunk. Getting drunk in many cases becomes the reason why they choose to drink a beer or even an expensive bottle of wine.
I grew up with a different mentality: in Italy it is not uncommon to see grandparents give a spoonful of red wine to kids to make ‘them relax’ or for treating their ‘sore throat’. It is surely not a wise choice but it’s not condemned as it is here.
Children in Italy do try wine before the age of 21 (or 18, the legal drinking age in Italy nowadays) at times at the hand of their own parents. It’s not seen as a big deal and therefore it does not become this ‘irresistible taboo’ alcohol is here.

I think that many are the campaigns for responsible drinking but they do not seem to work very well unfortunately.