Zooarchaeological analyses of pinniped remains have been scarce in South America because of lack of specific studies of species determination and estimation of age and sex. Nonetheless, the presence of small and unfused bones (assigned as pup remains) of Otaria flavescens (O. flavescens) in the Patagonian archaeological record has been frequently used as an indicator of summer occupation accomplished by ecological analogues. For this reason, we carried out a postcranial skeletons osteometric study of modern pups of South American sea lions (O. flavescens) with known sex and aged 0–12 months, allowing a more precise age estimation of archaeological samples. Then, we revisited the evidence for seasonality in the Cabo Virgenes 6 site at Cabo Virgenes locality, one of the archaeological localities in Patagonia where the seasonality has been determined only on the basis of the presence of unfused pup bones. Our metric data study on a modern sample shows that the regression analyses generated a complementary and dependable age estimation model for unfused appendicular bones of O. flavescens between 0 and 12 months old that can be applied to samples recovered from archaeological sites. These osteometric studies of postcranial elements allow us to adjust the age estimation of the animal's death and re‐examine seasonality of the Cabo Virgenes 6 site (Santa Cruz, Argentina). Copyright © 2013 John Wiley & Sons, Ltd.