Thirst isn't always a reliable early indicator of the body's need for water. Many people, particularly older adults, don't feel thirsty until they're already dehydrated. That's why it's important to increase water intake during hot weather or when you're ill. The signs and symptoms of dehydration also may differ by age. Infant or young child 
Dry mouth and tongue
No tears when crying
No wet diapers for three hours
Sunken eyes, cheeks
Sunken soft spot on top of skull
Listlessness or irritability
 Adult 
Extreme thirst
Less frequent urination
Dark-colored urine
Fatigue
Dizziness
Confusion
 When to see a doctor Call your family doctor if you or a loved one: 
Has had diarrhea for 24 hours or more
Is irritable or disoriented and much sleepier or less active than usual
Can't keep down fluids
Has bloody or black stool
