Abstract: Age--dependent metabolic model of radionuclides in human body wasintroduced briefly. These data are necessary in setting up the secondary doselimit of internal exposure of the general public. For the gastro--intestinal tractmodel, it was shown that the dose of various sections of GI tract caused byunsoluble radioactive materials were influenced by the mass of section and meanresidence time, both of which are age--dependent, but the absorption fraction f_1through GI tract should be corrected only for the infant less than 1 year of age.For the lung model, it was indicated that the fraction of deposition or clearanceof particles in the different compartments of lung were related to age. The dosesof tracheobronchial and pulmonary compartment of adult for ~(222)Rn or ~(220)Rn withtheir decay products were one third of that of 6-years old child who received themaximum dose in comparison with other ages. The age--dependent metabolic modelsin organ and/or body of Tritium, Iodine--131, Caesium--137, radioactive Strontium,Radium and Plutonium were reported. A generalized approach for estimating theeffect of age on deposition fractions and retention half--time were presented.Calculated results indicated that younger ages were characterized by increaseddeposition fraction and decreased half-time for retention. Representative exampleswere provided for 21 elements of current interest in health physics.