Wikipedia says 50:
In adults over the age of 50 years, the body's thirst sensation reduces and continues diminishing with age, putting this population at increased risk of dehydration.
And having to be stay at a hospital for a length of time for any reason is very much Not Good for an elderly person. Other illnesses, muscle atrophy, disorientation, loneliness, cognitive decline...
Hospital -> Social home, then what you have said ... and then death.
As someone who has a relative working in a hospital and another in a social home (for the elderly, but these days we get 50 years olds with dementia which is crazy), they experience it.
It is not necessarily about dehydration, but that plays a huge role, too. Sometimes a simple yet unfortunate fall is enough.
My discovery was Pedialyte it's meant for children but it's like the adult version or Gatorade. I drink it before I exercise and also drink as needed. I feel normal no headaches not dehydrated.
edit: I also have hypothyroidism so my hypothalamus must also be crap at regulating my thirst maybe?
I think adults tell kid-me to drink more water was a way of trying to get me to just develop a habit with it, since they understood the seemingly paradoxical struggle of keeping hydrated at their age.
Excessive thirst and urination is a potential symptom of diabetes, might want to get that checked out.