The deposition and retention models and the anatomical and physiological data as proposed by the ICRP Task Groups on Lung Dynamics and Reference Man are valid only for adult dosimetry. However, the change of the growing organism causes an age-dependent variation of the radiation burden to the respiratory tract. Therefore, age-dependent functions of anatomical and physiological parameters were defined. For this purpose data were either interpolated from literature or calculated from theoretical modeling. With these functions and defined aerosol composition, age-dependent deposition probabilities in the single regions of the respiratory tract were determined. For the demonstration of lung dosimetry as a function of age the naturally occurring radon daughters were used as an example. By applying typical mean nuclide concentrations found in the atmosphere of an urban environment and defined age-dependent daily life patterns, the annual inhaled amount of radioactivity was computed. With the above data, dose calculations for the single ICRP lung model compartments were performed. This revealed that the inhaled dose in both the tracheobronchial and pulmonary regions showed a strong dependence on age; a pronounced maximum value was reached at the age of about 6 yr for radon and thoron decay products.