I don’t know the cutoff age, but people who are “seniors” tend to not perceive “thirst” as well as they used to. So when dehydrating, they don’t impulsively drink fluids.
I got dehydrated a few years ago, and coupled with an adverse drug reaction to a new drug I just started taking, causing me nausea I ended up in the ER. Now, I make sure I am hydrated. If I feel a bit thirsty I make sure that I drink fluids ASAP...., v8, Gatorade, water, etc...
We swear by packets of Liquid IV (electrolyte powder).
I mix one up before working in the yard, taking a walk, etc. usually another one on a super hot day and I am taking a break. In fact, I’m in Maryland amd heat index is 100 and I just came in for a break and took another one! They work so much better for us than Gatorade or powerade. Pedialyte Plus is good, tho.