Er, if this is true, please explain this.

(and a copy/paste for the lazy)

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.

Are the oldest rocks in the world exposed at Grand Canyon?

No. Although the oldest rocks at Grand Canyon (2000 million years old) are fairly old by any standard, the oldest rocks in the world are closer to 4000 million years old. The oldest exposed rocks in North America, which are among the oldest rocks in the world, are in northern Canada.