It is true that as the brain ages, there are resultant physiological changes affecting the body.  Reaction time slows.  Some memory loss occurs.  It is not true that the brain is unable to manufacture new synapses and neurons: continuing education, developing hobbies, improving hand eye coordination, exercise and diet all play a role in aging gracefully.  That's the best case scenario.

What happens, though, if the brain starts to run amok? It's been roughly 100 years since Alois Alzheimer first diagrammed the pathology he saw, but it still defies the most brilliant minds trying to find a cure.  It most often occurs after age 60, and up to half of those over the age of 85 may be exhibiting some symptoms of Alzheimer's.  How do you know if what you are seeing is normal aging or pathology which needs to be addressed?