Here's why:

* Mid-ocean ridges are where new oceanic crust is formed. Magma rises from the Earth's mantle, cools, and solidifies, creating new crust.

* Seafloor spreading: As new crust is created, older crust is pushed away from the ridge on either side.

* Age gradient: This process creates a predictable age gradient. Rocks closest to the mid-ocean ridge are the youngest, while rocks further away are progressively older.

Therefore, the closer a rock sample is to a mid-ocean ridge, the younger it is.