How much water should elderly people drink a day?

If not sick... A simple equation to help adults figure their fluid needs is that for every pound of body weight, you need about half an ounce of fluid intake per day.  for instance, if you weigh 140 lbs., simply multiply 140 by .5 to estimate your daily fluid needs in ounces, then divide by 8 to estimate your fluid needs in cups per day, rounding up to the nearest full cup.
More than they do. Ideal fluid intake will depend on what disease states are present, what medications they are on, are there reasons to restrict fluid (such as hyponatremia), how active vs sedentary the person. A healthy active adult without significant medical problems: body weight in pounds divided by 2, then subtract 20, in ounces. So, dr k, at 170, should be drinking 65 ounces (my own concoction) per day.