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Notes:
Throughout life, use of an organ causes a gain of capacity; disuse causes loss of capacity. This is best understood in terms of muscle-building. But use also builds bone strength, increases flexibility of sinews, and -- perhaps to our surprise -- improves the brain.
In general, each body system has the capacity to increase its capacities and repertoire with use, and with disuse shuts down unused cellular mechanisms.
Examples: enzyme induction in the liver for metabolic processing
immune response
The brain adapts to use also, creating new circuits in response to learning and to the exercise of its faculties. This has been shown to occur in old age as well as youth (although I read the research paper in which this is shown a couple of years ago and don't recall the reference).
A consequence of aging is that this occurs less readily in older persons than in younger, so motor learningor abstract learning is much faster in the young. But there is no point at which learning is known to turn off.
There is a wide range of ability among people. This range is much greater than the range through which a normal person's ability changes as he or she ages.