Brain function varies normally as people pass from childhood
through adulthood to old age. During childhood, the ability to
think and reason steadily increases, enabling a child to learn
increasingly complex skills. During most of adulthood, brain
function is relatively stable. After a certain age, which varies
from person to person, brain function declines. Different
aspects of brain function are affected at different times:
However, the effects of aging on brain function may be difficult
to separate from the effects of various disorders that are
common among older people. These disorders include depression,
stroke, an underactive thyroid gland (hypothyroidism), and
degenerative brain disorders such as Alzheimer's disease. As
people age, the number of nerve cells in the brain usually
decreases, although the number lost varies greatly from person
to person, depending on the person's health. Also, the remaining
nerve cells function less well. However, the brain has certain
characteristics that help compensate