A method is presented for estimating relative age, regardless of size, in a group of fossil reptiles. Unequivocal evidence of relative age independent of size is available for Stenopterygius, a Jurassic ichthyosaur, since gravid females have been found. Osteological features associated with sexual maturity or immaturity in these specimens can then be used to estimate relative age in specimens unassociated with foetal individuals, individuals abnormal with respect to age and size can thus be recognized. In addition, age distinctions can be made in very small samples.Comparison of these features with stages described in a neontological model of growth supports their interpretation as products of a similar growth pattern. Extrapolation from the model allows exploration of the relationships between sexual maturity, osteological maturity, and size. Results indicate that sexual and osteological maturity were not coincident in Stenopterygius, with sexual maturity occurring prior to osteological maturity. Relative age distinctions can therefore be made between old and young individuals within the category of sexual maturity. Osteological maturity is related to relative size within a species. It is independent of absolute size. Accordingly, interspecific comparisons of relative age can be made regardless of the size of the specimens compared.