There are a range of scientific estimates of the Canyons age. Either it's about 5 million years old, as indicated by a sedementary sequence to the west of the Colorado Plateau, or it's 17-20 million years old, as that's when the Plateau started uplifting, and so when the river would start cutting down.

The river will have changed course in response to the uplift of the plateau, so at some point the main channel will have been in what is now a side canyon, and different course change the age estimate. Trying to sort out what happened is rather like trying do do a crypic crossword, with most of the clues missing...