what happens to the brain as an individual grows older?
a. what happens to the billions of nerve cells in the cortex as one becomes older?
b. will the dead neurons be replaced?
c. why is memory affected as one grows old?
Many of these questions have no answer but here is a stie that will give some insight.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596698/