Why is continental crust usually older than oceanic crust?

On balance, the age of the continental crust (of a continent) will be older than the oceanic crust because the continental crust is not being recycled (to the extent that the oceanic crust is recycled)! To understand this phenomena you have to first understand where the oceanic crust is formed and where it subducts.

Explanation:

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Continental crust rarely gets destroyed and recycled in the process of subduction. Oceanic crust gets colder and denser with age. It gets dense to the point that it sinks in the upper mantle. The continental crust is lighter than the oceanic crust. The continental crust doesn't subduct. 

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