Major variations in venom composition can occur between juvenile and adult venomous snakes. However, due to logistical constraints, antivenoms are produced using adult venoms in immunising mixtures, possibly resulting in limited neutralisation of juvenile snake venoms. The Russell’s Viper (Daboia russelii) is one of the leading causes of snakebite death across South Asia. Its venom is potently procoagulant, causing in humans consumptive coagulopathy, a net anticoagulant state, and sometimes death resulting from hemorrhage. In this in vitro study, we compared the venom activity of—and antivenom efficacy against—six 2-week-old D. russelii relative to that of their parents. Using a coagulation analyser, we quantified the relative coagulotoxicity of these venoms in human, avian, and amphibian plasma. The overall potency on human plasma was similar across all adult and neonate venoms, and Serum Institute of India antivenom was equipotent in neutralising these coagulotoxic effects. All venoms exhibited similar potency upon avian plasma. In contrast, neonate venoms were more potent on amphibian plasma, suggesting amphibians make up a larger proportion of diet in neonates than adults. These results suggest a novel ontogenetic variation: instead of toxin classes varying between life stages, the results suggest the presence of different toxin isoforms. Overall, these results inform the clinical treatment of patients envenomated by this deadly species, and the data also shed new light on the natural history of these extremely medically important snakes.