The signs and symptoms of dehydration in an elderly person can be virtually identical to those for dementia. The most common signs and symptoms of dehydration include persistent fatigue, lethargy, muscle weakness or cramps, headaches, dizziness, nausea, forgetfulness, confusion, deep rapid breathing or an increased heart rate. Because seniors often have a reduced sense of thirst, dehydration is one of the most frequent causes of hospitalization after the age 65. Make sure the senior in your life is eating enough and drinking the daily recommended amount of water.