We're all familiar with the concept of interpretation. We can all look at the same event, the same real event, and based on the sum total of our experiences and knowledge, view it through our own particular lens.
The really good artists are the ones who get us to see their perspective and realize things we missed. That's art, and it doesn't matter if the artist is five or Edward Hopper. What makes Hopper Hopper is his art affect us on an emotional level enough that someone wants to buy it.
But there's a larger point I'm missing, and that is for the culture at large. As JohnR in comments put it:
I've used a simple definition of "art" for most of my life, reinforced by a recent visit to our local MoA with the wife: Art is anything that someone thinks is art. Do something; think it's art? Then it's art. It helps if you can convince somebody else that it's art, which can be surprisingly easy, but that's optional.
It occurs to me that art is a communication, as I pointed out above, but like all communications, it is subject to individual accents and understandings. A rock is a communication to someone, somewhere.
And I mean before it's even thrown at your head. It provides information, for one thing: there are likely other rocks nearby, which could mean there's a large rock either under the ground or up on a hillside.