While the body changes with the passing years, seniors have basically the same water needs as adults younger than themselves.
Change at a cellular level
Muscles atrophy with age and seniors consequently have less water in the body than younger adults. On average, from infancy to old age, the body’s water content diminishes from 75% to 50% water. The kidneys’ ability to reabsorb water decreases with age, so water is lost from the body in greater amounts.
Taking special care
Since the sensation of thirst diminishes as they grow older, seniors need to pay close attention to their water intake, to avoid dehydration(1). By the same token, elderly people feel less hungry and eating less food reduces hydration from this source.
Older people are also at higher risk of illness and serious injury. Should either occur, adequate water intake is vital to prevent dehydration resulting from fever and vomiting/diarrhea, blood loss or wound secretion.
Sources: (1) Jéquier et al. Water as an essential nutrient: the physiological basis of hydration. EJCN 2010, 64: 115-23