We are a chronically under-hydrated society. And, as we age, it becomes even more important to keep hydrated. (Me, sitting here, knowing how much fluid I didn't drink today...) Maybe that's (one of the reasons) why many elderly people get dementia.
If you start to feel the sense of thirst, it is already too late - you are not hydrated. We are told in the health care sector to ensure you drink enough. Caffiene-filled drinks are only worth about half the fluid in it, as caffeine is a diuretic - for you not in the health care field, that means it makes you pee, just like beer!
When you have the flu, it's vitally important to keep drinking, even if it just.... goes through you are doesn't stay in your system very long.
We are too kind to the drunks who come into the hospitals. We give them fluid intravenously. A hang-over means dehydration AND electrolyte loss. Fix them up by giving them both, and they don't get hung-over.
Now I need Vexxie to edit, as I am babbling educating the masses. And now, I'll get off my soap box and get something to hydrate by brain!