Measurement of skull ossification patterns is a standard method for aging various mammalian species and has been used to age Russian, Californian, and Alaskan sea otter populations. Cementum annuli counts have also been verified as an accurate aging method for the Alaskan sea otter population. In this study, cementum annuli count results and skull ossification patterns were compared as methods for aging the northern sea otter (Enhydra lutris kenyoni) population in Washington State. Significant agreement was found between the two methods suggesting that either method could be used to age the Washington population of otters. This study also found that ossification of the squamosal-jugal suture at the ventral glenoid fossa can be used to differentiate male subadults from adults. To assist field biologists or others without access to cementum annuli or skull ossification analysis techniques, a suite of morphologic, physiologic, and developmental characteristics were analyzed to assess whether a set of these more easily accessible parameters could also predict age class for the Washington population of otters. Tooth condition score, evidence of reproductive activity in females, and tooth eruption pattern were identified as the most useful criteria for classifying Washington sea otters as pups, juveniles, subadults, or adults/aged adults. A simple decision tree based on characteristics accessible in the field or at necropsy was created that can be used to reliably predict age class of Washington sea otters as determined by cementum annuli.