Epibionts can serve as valuable ecological indicators, providing information about the behaviour or health of the host. The use of epibionts as indicators is, however, often limited by a lack of knowledge about the basic ecology of these ‘hitchhikers’. This study investigated the growth rates of a turtle barnacle, Chelonibia testudinaria, under natural conditions, and then used the resulting growth curve to estimate the barnacle’s age. Repeat morphometric measurements (length and basal area) on 78 barnacles were taken, as host loggerhead turtles (Caretta caretta) laid successive clutches at Mon Repos, Australia, during the 2015/16 nesting season. Barnacles when first encountered ranged in size from 3.7 to 62.9 mm, and were recaptured between 12 and 56 days later. Fitting the growth measurements of these barnacles to a von Bertalanffy growth curve, we estimated the age of these barnacles as a function of their size. Length growth rate decreased over time in a non-linear fashion, while basal area growth rate showed a linear relationship with age. The average estimated age of barnacles at first capture was approximately 6 months (barnacle length 30.3 ± 1.8 mm). The largest and oldest individual had a length of 62.9 mm when first measured, and was estimated to be 642 days old. The finding that C. testudinaria may live for up to 2 years, means that these barnacles may serve as interesting ecological indicators over this period. In turn, this information may be used to better understand the movement and habitat use of their sea turtle hosts, ultimately improving conservation and management of these threatened animals.