Final answer:
The lowered brain volume in older adults compared to younger adults is primarily due to a lower number of neurons.
Step-by-step explanation:
The primary reason for the lowered brain volume of older adults compared to younger adults is D) lower number of neurons. As individuals age, there's a natural loss of neuronal cells, which contributes to the overall decrease in brain volume. Contrary to the growth of dendrites and the increased complexity of axons, which are typically associated with cognitive development and learning capability, the physical atrophy of the brain due to neuron loss is a hallmark of aging. Neurodegeneration, reflected in conditions like reduced corticostriatal connections and hippocampal dendritic atrophy, has been implicated in this process, affecting cognitive functions and spatial representations in older adults.