What is the mechanism of Brain Volume reduction in aging?

It is commonly seen in the MRI's of elderly or aging individuals that there is significantly reduced brain volume (Takao et al. 2012) and enlarged lateral ventricles (Scahill et al. 2003). The mechanism of these two changes in the brain seem to be coupled in that the lateral ventricles are a space-filling structure that contain fluid which exert pressure on the bounding walls, whereas the brain tissue lies just outside these bounding walls and prevent outward expansion.

Question: Why does brain volume decrease in aging individuals and how is this decrease in brain volume coupled to increases in lateral ventricle volumes? Which of these anatomical changes occurs first?