Neurons and aging brains: what we know

Claims that the brain starts with “14 billion cells,” loses 100,000 neurons every day after age 25, or suddenly shrivels after 40–50 are not reliably supported. An estimate for the adult brain is about 86 billion neurons, with natural variation between individuals. A healthy brain may shrink somewhat with age, but this does not mean massive neuron loss; age-related changes differ across regions and people. Whether new neurons form in particular regions of the adult human brain remains under study.