Here's why:

* Seafloor Spreading: Mid-ocean ridges are the sites of seafloor spreading. Magma rises from the Earth's mantle and erupts, creating new oceanic crust.

* Continuous Process: This process of magma rising and creating new crust is continuous. As the new crust forms, it pushes older crust away from the ridge.

* Age Gradient: This process results in an age gradient in ocean basins. The youngest rocks are found at the ridge, while the oldest rocks are found furthest away from the ridge.

So, the youngest rocks in an ocean basin are constantly being created at the mid-ocean ridges.