The aging brain partly compensates for a loss of brain cells by recruiting and reorganizing existing neural networks. this best illustrates

Plasticity

As a person ages, its brain may experience progressive loss in function that includes sensation, memory, motor control and cognition. This may manifest throughout the different system of the body due to wearing down over time. One effect is called neuro-plasticity which illustrates the ability of the nervous system to not function well due to loss brain cells. This results from responding to intrinsic and extrinsic stimuli, which consequences to reorganizing its structure, function, and connections.  

see below  

explanation:

all of the above