Hydration and Aging: Why Your Water Needs Change Over Time

Hydration is a lifelong concern, but the way your body manages water changes dramatically from infancy through old age. A 5-year-old, a 35-year-old, and an 85-year-old all have different hydration needs, different risk factors, and different warning signs to watch for. This decade-by-decade guide explains how hydration changes across the lifespan and offers practical strategies for every life stage.

Infancy (0 to 12 Months)

Infants have the highest proportion of body water of any age group — about 75 percent of body weight, compared to 60 percent in adults. Their kidneys are immature and cannot concentrate urine effectively, which means they lose water rapidly and are highly vulnerable to dehydration. Breast milk or formula provides all the water an infant needs for the first 6 months — giving plain water to infants under 6 months can actually be dangerous because it can cause water intoxication (hyponatremia).

After 6 months, small amounts of water can be introduced along with solid foods, but breast milk or formula remains the primary fluid until 12 months. Infants are particularly vulnerable to dehydration during illness — vomiting, diarrhea, and fever can cause rapid fluid loss. Signs of dehydration in infants include no wet diaper for 3 hours or more, sunken soft spot (fontanelle) on the head, sunken eyes, dry mouth, crying without tears, and unusual sleepiness or irritability. If an infant shows signs of dehydration, contact a pediatrician immediately.

Childhood (1 to 12 Years)

Children need more water per kilogram of body weight than adults because they have a higher proportion of body water and a faster metabolic rate. The Institute of Medicine recommends about 1.3 liters per day for children 1 to 3 years old, 1.7 liters for children 4 to 8, and 2.1 to 2.4 liters for children 9 to 13, from all beverages and foods. Active children and children in hot climates need more.

Children often do not recognize or communicate thirst effectively, so parents and caregivers must encourage proactive drinking. Send a water bottle to school, offer water with every meal and snack, and make water the default beverage. Limit juice to 120 to 240 ml per day and avoid soda and energy drinks entirely. For more detail, see our guide to hydration for kids and teens.

Adolescence (13 to 18 Years)

Teenagers have hydration needs similar to adults — about 2.4 liters per day for girls and 3.3 liters for boys, from all sources. Athletic teenagers need more, particularly during training and competition. Teenagers are also at risk of dehydration from sugary drink consumption, energy drink abuse, and (for some) eating disorders that involve fluid restriction. Encourage water as the primary beverage and educate teenagers about the risks of energy drinks and the importance of hydration for athletic performance.

Young Adulthood (19 to 39 Years)

Young adults typically have efficient hydration regulation and can rely on thirst as a guide under normal conditions. The recommended intake is about 3.7 liters per day for men and 2.7 liters for women, from all beverages and foods. Pregnancy and breastfeeding significantly increase needs — see our guides to pregnancy hydration and the Pregnancy Water Intake Calculator for details.

Young adults who exercise intensively, work outdoors, or live in hot climates need more water. Calculate your personalized target with our Daily Water Intake Calculator. Common hydration pitfalls in this age group include excessive caffeine consumption, alcohol, and relying on sugary energy drinks for hydration.

Middle Adulthood (40 to 64 Years)

Beginning around age 40, total body water begins to decline gradually, and the kidneys become slightly less efficient at concentrating urine. Thirst sensation may begin to weaken, though it is still reliable for most people in this age group. The recommended intake remains about 3.7 liters for men and 2.7 liters for women, but individual needs may vary based on health conditions, medications, and activity level.

Many people in this age group begin to take medications that affect hydration — diuretics for blood pressure, antihistamines, antidepressants, and others. Discuss your specific medication-related hydration needs with your healthcare provider. This is also the age group where chronic conditions like diabetes, kidney disease, and heart disease become more common, and these conditions can significantly affect hydration needs.

Young-Old Age (65 to 74 Years)

After age 65, hydration risk increases significantly. Total body water drops to about 50 to 55 percent of body weight, the thirst sensation weakens by about 30 to 40 percent, the kidneys become less efficient at concentrating urine, and medication use increases. Many older adults are chronically mildly dehydrated without realizing it. The recommended intake is about 2.5 to 3 liters per day, but proactive drinking is essential because thirst is no longer a reliable guide.

Strategies for older adults include keeping a water bottle within reach at all times, drinking at specific times (on waking, with meals, mid-morning, mid-afternoon) rather than relying on thirst, choosing beverages that are appealing (warm beverages like tea and broth may be more appealing than cold water), and eating water-rich foods like soups, fruits, and vegetables. Family members and caregivers should watch for subtle signs of dehydration like confusion, fatigue, and falls. For more detail, see our guide to hydration for seniors.

Old-Old Age (75+ Years)

Adults over 75 face the highest dehydration risk. Mobility limitations may make it difficult to get a drink when wanted, cognitive impairment may cause forgetting to drink, swallowing difficulties may make drinking uncomfortable, and polypharmacy (taking multiple medications) increases fluid loss. Dehydration in this age group is a leading cause of hospitalization and is associated with increased mortality.

For adults over 75, hydration requires active management by the individual, family members, and caregivers. Strategies include offering fluids frequently throughout the day, providing beverages in easy-to-hold cups with handles or straws, addressing swallowing difficulties with a speech therapist if needed, monitoring urine color and output, and being vigilant for signs of dehydration. Care facilities should have hydration protocols and track fluid intake for residents.

Some medical conditions common in this age group require fluid restriction rather than increased intake — heart failure, kidney disease, and liver disease with ascites may all require limited fluid intake. Always follow the specific guidance of the healthcare provider for these conditions. For more on hydration across the lifespan and related topics, explore our complete articles library.