Why is oceanic crust younger than continental crust?

It is due to the process of subduction; oceanic crust tends to get colder and denser with age as it spreads off the mid-ocean ridges. It gets so dense, that it sinks in the upper mantle (subduction). As the continental crust is lighter than the oceanic crust, the continental crust cannot subduct.

This is what i did 45-5=4040÷4=10so there are 10 white marbles but am not sure is just a question that am not sure