It looks like the NPS will have to update their Website
From the National Park Service FAQ page:
How old is the Canyon?
That's a tricky question. Although rocks exposed in the walls of the canyon are geologically quite old, the Canyon itself is a fairly young feature. The oldest rocks at the canyon bottom are close to 2000 million years old. The Canyon itself - an erosional feature - has formed only in the past five or six million years. Geologically speaking, Grand Canyon is very young.