Several studies report methods for determining the age of juvenile Queensland koalas (Phascolarctos cinereus adustus) but these are mostly based on data from captive populations, because observing the birth of koalas in their natural habitat is extremely rare. We identified the exact date of birth for two male joeys by initially observing one within minutes and the other within hours of their birth, at St Bees Island, central Queensland. Successive measurements of head length, as these individuals matured, were used to construct a growth curve for free-ranging juveniles. When tested, only one previously published growth curve (based on body mass) was able to accurately estimate the age of the two joeys. Both methods were then tested for precision using morphometric data for other juvenile koalas in the St Bees population. The estimation of age of juvenile koalas was considerably more precise when based on head length. These results demonstrate the inaccuracy that may be inherent in growth curves derived from captive animals and also show that estimates of age based on data from individuals in a particular population or locality may not be accurate throughout the range of a species.