Here's why:

* Seafloor Spreading: Mid-ocean ridges are where new oceanic crust is created. Magma rises from the Earth's mantle, erupts, cools, and solidifies, forming new crust. This process is called seafloor spreading.

* Age Gradient: As new crust forms, it pushes older crust away from the ridge. This means the farther you are from the ridge, the older the rocks are.

* Subduction Zones: The older oceanic crust eventually gets subducted back into the mantle at areas called subduction zones.

So, the youngest rocks are constantly being produced at the mid-ocean ridges, while the oldest rocks are consumed at the subduction zones.