Illustration of three different functions proposed for scaling of drug clearance in young children: maturation/allometric, body weight–dependent, and age-dependent scaling. All three methods predict reduced clearance for young/small children compared with CL = 100 · (WGT/70)^0.75 (dotted black line). A linear model, CL = 100 · (WGT/70)¹ (dashed gray line), extrapolates reasonably from adults to young/small children but underpredicts drug clearance for older/larger children. CL, drug clearance (relative to 70-kg adult); PMA, postmenstrual age (in weeks); WGT, body weight (in kg).