No subject

There have been a number of studies of neuron loss in aging.
It proceeds at different rates in different parts of the brain, 
with some parts showing hardly any loss at all. Even in 
different areas of the cortex the rates of loss vary widely,
but it looks like, overall, about 20% of the neurons are lost 
by age 60. 

Using the standard estimate of ten bilion neurons in the 
neocortex, this works out to about one hunderd thousand
neurons lost per day of adult life.

Reference:

"Neuron numbers and sizes in aging brain: Comparisons of
human, monkey and rodent data" DG Flood & PD Coleman,
Neurobiology of Aging, 9, (1988) pp.453-464.
 

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