A somewhat gloomy citation from Christian De Duve, Vital Dust (Chapter 26, Brain, p. 240):
The human brain has completed all the neurons it will ever make some five month before birth. Contrary to what happens to other cell types, multiplication of neurons ceases after that. Henceforth, neurons only die, starting in utero, to the tune of hundreds of thousands per day. I have lost several billion neurons since I was born. Between starting and finishing this sentence, I have lost about one hundred more. The thought is unsettling, but I take comfort in assurance from my neurobiologist friends who tell me that many brain connections are superfluous and redundant and that, even though I cannot replace my neurons, I can still rewire some connections if I keep sufficiently busy.