
“Minor may also be used in contexts that are unconnected to the overall age of majority. For example, the smoking and drinking age in the United States is 21, and younger people below this age are sometimes called minors in the context of tobacco and alcohol law, even if they are at least 18” from Wikipedia. Legally you are still a minor under 21 in the contexts of alcohol
In addition, the actual “no minor” signage actually specifies people under 21, usually using language like “no person under the age of 21 may be permitted to enter” at least on the signs in my area. Either laws are different where you are, the places you went to are lenient, or you were only allowed because you were with your parents. Very likely you would not be allowed without them