If you shop at a wine or liquor store, or you dine at a restaurant and order some alcohol, there is the possibility that you will be carded, asked to provide identification to prove your age. You must be at least 21 years old to obtain alcohol in any state in the U.S. If you lack identification, you will be refused service, no matter how old you may be. If they wanted, restaurants and wine/liquor shops could card every single customer. If they fail to card someone who is a minor, they could face sanctions and even potentially lose their liquor license.
When I card customers at the wine shop, the vast ,majority of customers provide me identification without issue. They understand that it is part of my job and there is absolutely no reason to take offense. There is a tiny portion of people though who do not have any identification on them, and who I must refuse service. It is a portion of these individuals who get upset about being carded, who complain, alleging that they are of the proper age. They might be of legal age, but without identification, there is nothing I can do. I must not sell them beer or wine.
What bothers me is why don't these people carry identification?
I don't think all of these people are underage. I'm sure some are, and bluster and storm hoping I will sell them alcohol anyways. However, I bet some of them are of age, yet simply didn't carry a ;license, passport or other i.d. with them. These same people carry money and/or credit cards, so why don't they carry i.d. too? It seems to make no sense, especially if you are going somewhere where you potentially could be asked for your i.d.
I know that I carry i.d. with me all the time, and it seems most people I know do the same. I.D. can be important for numerous reasons. Heaven forbid you get into a terrible accident and are unconscious. If you have an i.d., your relatives can more easily be contacted by the authorities. Without that i.d., the authorities could have a difficult time trying to identify you. If you are driving, you could get into trouble if the police stop you for some traffic offense.
A license isn't a cumbersome item to carry. There really is very little reason, if any, not to carry it with you. It could save you much grief, in numerous ways. Don't complain if you are asked for your i.d. if you don't have it. The onus is on you to carry it.