pretty much agree with jg.....
.....while 8 glasses might be too much for some one of short statue, one the thin side etc.....I'm pretty sure some one taller (and people heavier than average weight) would probably need close to 8, to above 8 glasses a day maybe closer to the 10 mark.
Obviously exercise, higher temps-- you need to replenish during & after.
It flushes those environmental toxins out, helps various bio-chemical processes to take place properly, keeps the blood at a proper liquidity...... and other stuff i've read about ?ie"brain fog" .
I do my best to get the 8. Sometimes I am dehydrated for a bit/ a while esp IF i forget to take a bottle with me while I'm out, and about.
I'm mid-50's enjoy mostly good physical health, good blood etc test results, and good flexibilty- my problems for the most part have been with eye & ear stuff I was born with [not so good hearing, and
LOUD concerts rock&roll added trouble/ VERY near-sighted too].
*REPEAT* "ear-plugs are my friends"*! I have permanent ringing in one ear from a WHO concert from waaay back from whence people didn't wear them for concerts.
I can still be at a U2 or Springsteen concert for in stance, and spend most of the time on my feet & dancing by my seat.
Usually no aches & pains unless I've "done something" ie over-done or in wrong postition -
like I rarely <2x's a year to once every 2 years> have any back pain [but I HAVE had it, and pretty bad for short stints of time <hrs - a few days> -relatively speaking to many others]. Only when i sit too long in one particular way, and stretch out too (sometimes) much to get at stuff I'm organizing/ cleaning up does it have a chance of happening.
I usually get a warning sign- then I readjust my position/activities. So I've learned to mostly avoid it> it did sneak up on me 3 weeks ago [very unusual], but i had been doing a lot of reorganizing etc.
So among other things {good foods, walking a lot, some tai ch, having play-time & fun etc} I also attribute often drinking a healthy amount of water as part of my efforts to remain healthy. Some good genes, and luck have probably played a part in there for me, too!