The blotched blue-tongued lizard, Tiliqua nigrolutea is a long-lived (anecdotally 30+ years) viviparous reptile, in which males reproduce annually and females breed irregularly in Tasmania, the southern most part of their range. Sexual dimorphisms are seen in adults, in both body length and relative head width. Adult females are larger (longer and heavier), while males have relatively broader heads. We have examined sex-related differences in the timing of the onset of sexually dimorphic features (using growth rate and relative head width) and correlated them with changes in the ratio of plasma testosterone to estradiol to document morphological and physiological dimorphisms in age and size at maturity between the sexes in this spring breeding species. Juvenile males reach adult size in relative head width and body length measures by 2.5 (Spring) years of age, while females do not attain adult proportions until age 3 (Autumn). Physiologically, both basal circulating testosterone (T) and T to estradiol (E2) ratio in males resembled adult proportions at 2.5 years of age (Spring). Female T:E2 at 3 years of age (Autumn) do not resemble adult female T:E2 proportions. We suggest a sexual dimorphism in age at maturity in this long-lived, viviparous lizard.