We employ samples of children of known chronological age to demonstrate the significance of random and systematic effects on maturation in both dental and skeletal development. Differences between chronological age for dental age in young healthy Canadian children can be as much as 100% of the actual age of the children. For skeletal development by reference to Greulich-Pyle standards, three samples of known-age children from Mexico document parallel effects: 1) 183 six-year-old children have skeletal-based ages with a 95% confidence interval of 4-8 years; 2) 80% of 217 4.0-4.5-year-old children are underaged by 1-3 years; and 3) 130 children of skeletal age between 39 and 44 months are actually between 4 and 7.4 chronological years of age. The Mexican samples are drawn from a population living under conditions of environmental stress with chronic mild to moderate protein-energy malnutrition and moderate to high levels of infectious disease. These children may parallel those from the past, whose remains are studied by skeletal biologists or paleoanthropologists. Our findings reinforce concerns expressed in extant studies regarding the accuracy of age-at-death reconstructions.