I Have 2 Questions.

Since the proposal of 1 day short easily converts to only 1 day short of 1 day short, I believe the fallacy becomes one of slippery slope .  Usually, the context is the basis of a warning, but here it's behind the claim.

I don't see a fallacy in the second statement. It's part of an admonishment not to follow the friends in error

Q1

I see this as an issue of context, not a logical fallacy. Depending upon the context, being one day short of eighteen is not much different than being eighteen. BUT IF you are talking about the legal aspect, then the proposition becomes: One may not legally drink until age eighteen. You are not age eighteen. Ergo, you may not legally drink. 

The kid’s better argument would be that it is tomorrow in Australia. Therefore, he is eighteen. :)

Q2

I see this only as a question to the kid. He can answer yes or not.

Q1: You are referring to the argument of the beard . The example you present, however, is of a different structure so not fallacious (at least not in the same way).  Your example is more an issue with simply being wrong about the law.

Q2: No fallacy, that is not an argument. You could reword it to make an appeal to popularity , if the claim is that it would be the right thing to do.