Here's why:

* Seafloor Spreading: Mid-ocean ridges are where new oceanic crust is created. Magma from the Earth's mantle rises to the surface, cools, and solidifies, forming new crust.

* Continuous Formation: This process is ongoing, so the crust furthest from the ridge is the oldest, and the crust closest to the ridge is the youngest.

* Subduction: As new crust is formed, older crust moves away from the ridge and eventually gets pushed back down into the mantle at subduction zones.

Therefore, the youngest crust on Earth is constantly being formed at mid-ocean ridges.