For the second point its not an assumption, if not now at...
Art is not a reality it is a concept to people choose to believed in.
Lets first examine your definition "the expression of creative skill through a visual medium such as painting or sculpture." Breaking it down "the expression of creative skill through a visual medium" This is exactly what my definition is saying that something worth seeing and brings you happiness the other portion of your definition was showing examples but wasn't absolute as to what can be defined as art. Also you said "a set standard or have any one person or level of skill achieved by any one person that would universally be a considered art" Thats what I am saying there are not any standards to art but it ties in with point two where everyone has known happiness so everyone knows art. But all definitions meet this basic standard for art, that everything is art. For the second point its not an assumption, if not now at some point you were happy you found something wonderful that you couldn't take your eyes off. Whether 10 or 20 years ago. On the third point its all personal opinion but everyone knows art, you might think somethings hideous but I might say other wise and vice versa. But yes there are all different opinions but they are all on the same basic standard listed above. For this final point art is existent if only in our minds it still exists. Its proven anytime you see something and you think wow thats cool. To deny that it has never happened would be like saying the sky isn't blue