First, just to be clear, "the pentatonic scale" isn't a discrete, singular, unchanging entity. By definition, any scale featuring five distinct tones per octave is a pentatonic, regardless of the specific arrangement of pitches within. There are two major classes — hemitonic, which contains half-steps, and anhemitonic, which doesn't — each with several common variants, plus some oddities like slendro. Pentatonics are present in musics from all over the world and throughout recorded history. Traditional Japanese music alone uses half a dozen different pentatonic arrangements (pp. 80-81).

The oldest known musical instruments, such as the 35-thousand-year-old Hohe Fels flute that was unearthed in Swabia in 2008, are perfectly capable of playing a number of pentatonic scales. The Divje Babe flute, if it is indeed a flute, is ~43 thousand years old. While it's obviously impossible to reconstruct any actual musical practice from tens of thousands of years ago, given A) the physical capabilities of the prehistoric instruments we've discovered and B) their pervasiveness in world musical traditions, it's reasonable to assume that pentatonics are essentially as old as music itself.