That children are not just miniature adults is well understood by pediatricians. A newborn infant with congenital hypothyroidism will have permanent devastating neurodevelopmental effects if not quickly treated with thyroid hormone; older children and adults who acquire hypothyroidism do not experience similar brain damage. When administered to a newborn infant, the antibiotic chloramphenicol causes "gray baby syndrome," a condition that does not occur in older infants, children, or adults.
Etzel RA. Child Health and the Environment. Arch Pediatr Adolesc Med. 2004;158(2):188-189. doi:10.1001/archpedi.158.2.188