One reason why the gaur Bos gaurus is a poorly understood species is because there are no reliable data to age and sex individuals. We studied captive gaur for two years in Mysore Zoo, India and Omaha Zoo, USA, and determined age-specific differences in morphological features and physical growth, by measuring shoulder height, of male and female gaur. We fitted von Bertalanffy growth functions to the shoulder height data and found maximum shoulder heights of 175 cm and 147 cm for males and females, respectively. This study ascertained for the first time that the greater the amount of white on the horns of a gaur, the older it is, and that the sexes can be distinguished based on horn shape and size differences. We found that gaur aged 15 months and below can be classified into three age classes, but are difficult to sex. The sex of gaur aged 15–36 months is best determined by a study of their horns. Adult gaur, greater than three years, are easy to sex, but are difficult to classify into single-year age classes and are reliably classified only into two age classes for females and three for males.