Previous studies have shown that children have a higher aqueous fraction of the fat-free body mass (FFM) than young adults. In older adults, methodological differences among studies limit evaluation of potential age-related differences in the water content of the FFM (W/FFM). Therefore, we determined W/FFM in 28 healthy white prepubescent children (age = 5-10 yr), 31 young adults (age = 22-39 yr), and 62 older adults (age = 65-84 yr), using 2H2O dilution to estimate total body water and a multicomponent approach based on body density, total body water, and regional bone mineral density to estimate FFM. To quantify the extent to which variation in W/FFM affects percent fat estimation error, differences in percent fat between our multicomponent approach and the Siri two-component model were related to W/FFM. Prepubescent children (72.7 +/- 1.6%) and older adults (72.5 +/- 1.4%) were found to have significantly higher (P < 0.01) mean W/FFM than young adults (70.8 +/- 1.2%). Differences in percent fat between the multicomponent and two-component models ranged from -10 to 6% fat and were significantly associated with W/FFM (r = -0.62, P < 0.0001). We conclude that prepubescent children and older adults, on average, have a higher W/FFM than young adults, and that, in adults, individual differences in W/FFM account for a substantial portion of the percent fat estimation errors associated with the use of two-component body composition models.