Until recently, doctors regularly diagnosed retirees with a catchall condition called senile dementia. Senility, as defined in the dictionary, means old age. Physical and mental deterioration were presumed to be part and parcel of the condition. Some of that thinking was based on scientific beliefs about the human brain that have only recently changed.

Until the late twentieth century, scientists thought humans were born with all brain cells already formed, and that brain deterioration was normal as cells died off in large numbers late in life. The brain was thought to be hard-wired once it was fully developed, in late adolescence. Scientists have since learned that brain plasticity continues through adulthood and that it enhances cognitive reserve.

Stunning scientific discoveries during the 1990s showed that neurons can divide, propagate, and develop into functional new nerve cells in the brain. This process, called neurogenesis, is known to occur in the hippocampus (a center of learning and memory) and in a part of the brain involved with smell. This understanding has led to a radical rethinking of the function and dysfunction of the adult human brain.

Your brain reaches its maximum weight when you're about 20 years old and slowly loses about 10 percent of its heft over your lifetime. That happens because neurons shrink, and synapses — the connections between neurons — gradually deteriorate.

The brain's supply of neurotransmitters, or chemical messengers, diminishes and communication slows, impeding your ability to think, remember, and calculate quickly. In most cases, these changes are a mere nuisance. They don't interfere with your ability to function from day to day.