We can never know, but probably thousands of years earlier than the earliest artifacts archeologists can dig up. For an artifact to survive long enough to reach the hands of an archeologist, it must be made out of stone, ivory or some other such permanent material. I can only imagine that the first experiments in art happened with sticks in mud, or with charred bits of charcoal on faces and hands.
Löwenmensch (Lion Person), found in Hohlenstein-Stadel Germany, carved mammoth ivory 32,000 years old
What does this sculpture, uncovered in 1939 and created 32,000 years ago tell us our human ancestors? Just think, 32,000 years ago means we're talking about primitive hunter-gatherer cavepeople. The famous cave paintings of southern France, which you may have heard of, didn't happen for another couple thousand years. This is SO LONG AGO. And yet, the maker of this sculpture couldn't have been so different from us. The artist had an ability for abstract thought so refined that s/he could, from seeing some cave-lions in his/her life, form in his/her mind a generalized symbol representing "cave lion". S/he had such a firm grasp of this symbol that s/he could carve it into a piece of ivory, for others to recognize as a lion (even others who live 32,000 years in this artist's future, who know nothing about his/her culture). This would be mind-blowing enough, but the artist didn't just reproduce a lion, but combined ideas to create a lion standing like a human, in other words, an object of fantasy, playfulness and imagination. (As well as, perhaps, religious significance... we can't know for sure).
It seems like the maker of the Lion Person was nearly as smart and creative as we are today.
How little we really know about prehistoric society.
Let me intrigue you with one more example:
At an archeological site in Turkey called Göbekli-Tepe, archeologists have uncovered the vast remains of a prehistoric temple, perhaps the oldest on Earth (it's 6,000 years older than Stonehenge). It is made up of walls and megalithic stone pillars, carved with detailed scenes of animals by rivers (the first landscape art!). Until it's excavation starting in 1994, archeologists had always (quite logically) believed that only with the advent of agriculture, which made searching for food a less full-time job, that early people had enough free time to do things like structure themselves into towns and civilizations, or build large temples. That all makes a lot of sense... except that the temple at Göbekli-Tepe pre-dates agriculture in the region by a couple hundred years. This can only mean that, in prehistoric Turkey at least, the complex coordination required to build a temple came first, and perhaps even lead to the invention of agriculture. With this, our understanding of civilization is turned on its head.
Monolith from Göbekli-Tepe, Turkey, 11,000 years old