Old oceanic crust is more dense and COOL .
New oceanic crust is less dense and HOT .
At mid-ocean ridges two tectonic plates move apart and molten magma rises to form new oceanic crust. As this crust moves away from the ridge over time it gets older and cooler and its density increases. Density is related to the temperature because most solids (like rocks) expand a bit when heated and contract a bit when cooled.
Density = ##(mass)/(volume)##
So a decrease in volume upon cooling results in a higher density.
This is a really important concept because it explains why…
(a) the oceans are deeper away from mid-ocean ridges (the denser cool crust settles down to sit at a lower elevation on the Earth)
(b) the angle at which plates subduct into the mantle varies from place to place — compare Tonga (old cool crust sinks steeply so volcanoes and trench are close together on a map) with the Cascades (young warm crust subducts at a shallow angle so volcanoes are further away from trench).