Isabella d'Este 1499 Charcoal, Chalks and Yellow Pastel Leonardo da Vinci The Louvre
This may be considered the oldest pastel artwork extant. Go get 'em Leonardo da Vinci. An interesting backstory is given that Isabella may have been one of Leonardo's few female friends. He began his Mona Lisa (another subject) a handful of years later, circa 1503. That puts this work in the same vicinity of time, at the height of da Vinci's artistic powers.