Ages and Stages

The risk of experiencing a fluid imbalance increases with age. Why? Skeletal muscle mass declines, and the proportion of fat within the body increases. After age 60 years, water content drops to about 45%.

Likewise, the distribution of fluid within the body changes with age. For instance, about 15% of a typical young adult's total body weight is made up of interstitial fluid. That percentage progressively decreases with age.

About 5% of the body's total fluid volume is made up of plasma. Plasma volume remains stable throughout life.