Suppose on the highway, the speed limit is 55 mph. If I am on the highway driving, I am not communicating that I am driving 55 or less. Enough people drive more than 55 so that it is general knowledge that such inference is bullshit. Additionally, a person can tell when someone is driving way, way more than the limit from simple eyesight.
The restaurant bar situation is hardly different. Many teens engage in underage drinking. This is a known thing. A teen who is underage drinking is not necessarily communicating to anyone that they are acting legally and in fact it is well-known that younger people engage in risky behaviors more often, which includes illegal behaviors. It is probably less well known, but teens tend to binge drink more than the general population. This means if you observe someone who is drunk and very young looking, it ought to cross your mind that they could be underage. I seriously doubt anyway that anyone underage and completely drunk is holding together any kind of semblance of maturity.