But when we get older, our cueing becomes faulted. In other words, when older people are denied fluid for 24 hours, their thirst is blunted. We sometimes drink when we shouldn't, causing dilution of our body minerals, or we don't drink when we should which causes dehydration. Medicines, particularly diuretics, and disease conditions often seen in older people make fluid consumption more than a casual affair. Neglect of symptoms and poor compliance with medications can be very dangerous.
Severe constipation can also complicate dehydration.
Our hunter-gatherer ancestors only had water to drink. Nowadays, however, it seems almost no able-bodied person drinks plain water. Fluid comes in various colors and temperatures, with varying bubbles and other complications. But plain water still claims many advantages. It has no calories, or salt, and it does its job of lubricating us just fine.