There appears to be little loss of neurons related to aging in brains not damaged by Alzheimer Disease. (Carper, Jean. Your Miracle Brain. NY: HarperCollins Publishers, Inc., 2000, pp 22-24)

The average person stands to lose about 10% of his/her brain weight over a lifetime, much of this due to shrinking of neurons. Use it or lose it; the more you use your brain the higher the ratio of synapses to neurons.(Howard, Pierce J., PhD. The Owner’s Manual for The Brain. GA: Bard Press, 1994, 2000, pp 193-200)