Assigning ages to lions (Panthera leo) requires the use of subjective and objective criteria, and is useful for conservation decision-making in that age distributions can be defined from which demographic profiles can be extracted. We collated all age assignment criteria and found that a constraint of most objective criteria is that they require Immobilized or dead specimens to measure. Furthermore, nearly all criteria used lions with assumed ages to construct relationships or narrative descriptions. We show that digital photogrammetry provides digitally-derived measures of shoulder heights similar to that of linearly derived measures. In addition, such shoulder heights did not differ between captive and free ranging lions, or between different regions in Africa. Variation in shoulder height is primarily associated with sex-specific age. Age, using the von Bertalanffy growth curve, explained 92% and 97% of the variation in female and male shoulder height, a skeletal measure not strongly affected by resource availability. Simulations suggest that age assignment is relatively accurate for females and males with shoulder heights up to 70 cm and 95 cm, respectively. Thus for lions younger than two years of age objective criteria gives most precise estimates, while subjective criteria are more suitable for older lions. Key words: African lion, age assignment, growth, shoulder height.