Understanding the human development of pulmonary airspaces is important for calculating the dose from exposure to inhaled materials as a function of age. We have measured, in vivo, the airspace caliber of the small airways and alveoli at their natural full distension (Total Lung Capacity, TLC) by aerosol-derived airway morphometry (ADAM) in 53 children of ages 6 to 22 years and 59 adults age 23 to 80 years. ADAM utilizes the gravitational settling time of inhaled inert particles to infer the vertical distance necessary to produce the observed loss of particles to the airway surfaces at sequential depths into the lung. Previously, we identified anatomical features of the lung: EAD_trans, the caliber of the transitional respiratory bronchioles; EAD_min, the mean linear dimension of the alveoli; and VLD_trans, a measure of conducting airway volume. In the current study we found that EAD_min increased with age, 184 µm at age 6 to 231 µm at age 22, generally accounting for the increase in TLC observed over this age range. EAD_trans did not increase with age or TLC, averaging 572 µm, but increased primarily with subject age and height when the entire age range of 6 to 80 years is included (EAD_trans (µm) = 0.012([height(cm)] X [age(years)] + 508, p=0.007). VLD_trans scaled linearly with lung volume, but VLD_trans relative to TLC did not change with age, averaging 7.04 ± 1.55 % of TLC. The data indicate that from childhood (age 6 years) to adult, a constant number of respiratory units is maintained while both the smallest bronchioles and alveoli expand in size to produce the increased lung volume with increased age and height. Funded by USEPA Cooperative Agreement CR829522 and does not necessarily reflect EPA policy.