The sediments of the seafloor, the further they are from the ocean ridge, the older they are, and vice versa, the closer they are to the ocean ridge, the younger they are.
This redistribution of the age of the seafloor is due to the way the new crust forms. The new crust is forming where there's a divergent tectonic boundary between two oceanic plates. Because they leave a gap between them, the magma from the mantle manages to come to the surface. As it comes to the surface it starts to create the ocean ridge, which is essentially an underwater mountain range. Because the magma is constantly pushing and more and more material is coming out, the seafloor i pushed sideways constantly, thus the oldest sediments are the furthest, and the youngest are on the ocean ridge itself.
