The population database contains physiological information starting from newborns (age "0") to 80 year old individuals. In addition, a preterm model has recently been developed that has been integrated as a special population (see below). When a pediatric or elderly population is simulated, only the age dependence of anatomical properties (size of the organs, blood flow rates, body composition, etc.) is considered. However, important pharmacokinetic differences in children and elderly can be caused by age-related differences in, for example, hepatic metabolic activity. These effects are automatically accounted for if the ontogeny of the elimination process has been defined in the Expression tab for the underlying Individual. To manually account for variations in elimination, use the User Defined Variability tab.