Skull ossification and development data from skull specimens of male sea otters, Enhydra lutris kenyoni, collected from 1989 to 2011 that were used to categorize specimens into age classes as part of an investigation to refine aging criteria and methods for sea otters in Washington. Each specimen has an assigned accession number from the U.S. Geological Survey – National Wildlife Health Center (NWHC). *Criteria used in both the established and alternative skull-aging methods. **Additional criterion used in the alternative skull-aging method. Cranial sutures become obliterated as animals age, and thus were scored as evident, obscured, or obliterated. The rostral lip of the glenoid fossa, which contributes to the mandibular condyle lock described by Morejohn et al. (1975), was scored as absent, present, or prominent, and was considered prominent when it reached or exceeded the level of the adjacent caudal glenoid fossa. Characterization of the closure of the squamosal–jugal suture allowed for expansion of the male subadult age class.