I remember the same. As I approach 50, I do notice that I feel much more tired and mentally foggy when I fail to remember my daily water bottles. I've had to develop specific routines that help me remember to fill/drink my water bottle which goes pretty much everywhere with me. For being such a necessity, it sure is an oddly hard thing to remember to do.

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.