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Why does old oceanic crust sink at subduction zones?

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Earth's lithosphere and upper mantle — oceanic crust A cross section of Earth's outer layers, from the crust through the lower mantle.

Old oceanic crust sinks at subduction zones primarily because it becomes increasingly dense as it ages.12

As oceanic crust moves away from mid-oceanic ridges where it is formed, it undergoes thermal contraction and cools. Over time, this cooling process makes the oceanic lithosphere denser than the underlying [asthenosphere]. When this dense, older crust encounters a continental plate or a younger, warmer oceanic plate, it tends to dive beneath the more buoyant material.3421

Additionally, the subduction process is facilitated by metamorphic changes within the descending slab. As the crust descends, it experiences high pressures and relatively low temperatures, causing rocks to recrystallize into denser forms, such as [eclogites]. The formation of these high-density minerals provides an important additional force that pulls the slab deeper into the [mantle]. This continuous recycling of oceanic crust into the mantle is a fundamental aspect of plate tectonics, balancing the creation of new crust at spreading centers.52