* Oceanic crust is constantly being created and destroyed. New oceanic crust is formed at mid-ocean ridges, where magma rises from the Earth's mantle and cools. This process is called seafloor spreading. Older oceanic crust moves away from the ridges, eventually subducting back into the mantle at convergent plate boundaries.

* The oldest oceanic crust is about 200 million years old. This is because older oceanic crust is constantly being recycled through the process of subduction.

* Continental crust is much older. Continental crust is less dense than oceanic crust, so it doesn't subduct as easily. This means that some continental rocks are billions of years old.

So, while some oceanic rocks are billions of years old, most of them are much younger due to the ongoing cycle of seafloor spreading and subduction.