The youngest rock is always right at the ridge (where fresh magma keeps producing new rock) and gets older the farther away it is from the ridge on either side. And scientists can date the age accurately thanks to the Earth's magnetic field, which reverses itself every so often, as it has throughout the planet's history. That magnetic polarity gets locked into a newly formed rock as it cools and solidifies. By tracking the sequence of polarity flips in the rock near these ridges, researchers are able to tell how old a given section of the seafloor must be.

Source: Los Angeles Times February 07, 2018 21:00 UTC