Determining the population dynamics of the vulnerable Grey-headed Flying-fox, Pteropus poliocephalus requires accurate methods of estimating the age of wild animals. Traditionally, wild juvenile P. poliocephalus have been aged by comparing their forearm measurements to those of known-aged captive bred juveniles. To determine the degree of plasticity in the forearm growth rate and hence the reliability of this method, the forearm growth of two groups of captive-bred flying-foxes whose mothers were fed different diets was compared. The difference in diet, a protein supplement of pollen rather than milk powder, made no significant difference in the average length of the forearm at birth, but there were significant differences in the subsequent growth of each growth parameter tested, with the pollen supplemented maternal diet providing faster post-natal growth than the milk supplemented maternal diet. Maternal diet was also significantly correlated with a difference in the sex ratio with a majority of females being produced on the milk supplemented maternal diet and a majority of males being produced on the pollen supplemented maternal diet. There was no significant difference in the post-natal forearm growth of males and females independent of their mother's diet. The sigmoidal curve that best described the average post-natal growth rate of mother-reared P. poliocephalus was the Logistic Model Function. As the diet of wild flying-foxes varies from year to year it is likely that the growth rates of wild mother-reared flying-foxes vary from year to year along with the age of competent flight and independence. The plasticity in the forearm growth rate indicates that estimating the age of wild juvenile flying-foxes using growth rates of captive mother reared juveniles is unlikely to be an accurate method of ageing wild juvenile flying-foxes and other methods of ageing these animals should be investigated.