* Seafloor Spreading: The ocean floor is constantly being renewed through a process called seafloor spreading. At mid-ocean ridges, magma rises from the Earth's mantle, creating new oceanic crust. This new crust pushes older crust away from the ridge, like a conveyor belt.
* Subduction: As the older crust moves away from the ridge, it eventually reaches a subduction zone, where it dives back down into the mantle. This process destroys the old crust.
Therefore, the oldest oceanic crust will be found farthest from the mid-ocean ridges, where it has had the most time to move away before being subducted.
Keep in mind that even the oldest oceanic crust is still relatively young compared to continental crust, which can be billions of years old.
