The "fervently debated" age of Grand Canyon is a hot topic again today with advance online publication in Nature Geoscience of a paper by Karl Karstrom and colleagues, titled "Formation of the Grand Canyon 5 to 6 million years ago through integration of older palaeocanyons."
The authors say that "although parts of the canyon are old, we conclude that the integration
of the Colorado River through older palaeocanyons carved the Grand
Canyon, beginning 5–6 million years ago."
You can listen to the NPR interview with the authors at http://www.npr.org/2014/01/27/265437261/grand-canyon-may-be-older-and-younger-than-you-think