Telomeres are the protective caps at the ends of all eukaryotic chromosomes. Because DNA replication of chromosome ends is incomplete, telomeres undergo sequence loss with each cell division resulting in the progressive shortening of their lengths. Telomere shortening with age is known from terrestrial mammals. We test whether this pattern is shared by marine mammals, by comparing telomere lengths between age classes in a pinniped species, the Australian sea lion (Neophoca cinerea). Telomere lengths were measured using a real-time quantitative polymerase chain reaction (PCR) method in specimens from three age classes: pup (<1.5 yr), juvenile (1.5–5 yr), and adult (>5 yr). Mean telomere lengths of the adults were significantly shorter than the juvenile and pup classes. However, we were unable to differentiate between pups and juveniles. These findings confirm that the Australian sea lion shares the general pattern of shortening telomere lengths with age as documented in terrestrial mammals. The application of telomere lengths as an age determinant requires considerable development to refine the scale of the age estimates derived, which will require the use of known-aged individuals. Nonetheless, measures of telomere lengths have the potential to become valuable tools in molecular ecology and forensics for assessing compliance in harvesting situations.