Summary 1. For air-breathing animals in aquatic environments, foraging behaviours are often constrained by physiological capability. The development of oxygen stores and the rate at which these stores are used determine juvenile diving and foraging potential. 2. We examined the ontogeny of dive physiology in the threatened Australian sea lion Neophoca cinerea. Australian sea lions exploit benthic habitats; adult females demonstrate high field metabolic rates (FMR), maximize time spent near the benthos, and regularly exceed their calculated aerobic dive limit (cADL). Given larger animals have disproportionately greater diving capabilities; we wanted to determine the extent physiological development constrained diving and foraging in young sea lions. 3. Ten different mother/pup pairs were measured at three developmental stages (6, 15 and 23 months) at Seal Bay Conservation Park, Kangaroo Island, South Australia. Hematocrit (Hct), haemoglobin (Hb) and plasma volume were analyzed to calculate blood O 2 stores and myoglobin was measured to determine muscle O 2 . Additionally, FMR's for nine of the juveniles were derived from doubly-labelled water measurements. 4. Australian sea lions have the slowest documented O 2 store development among diving mammals. Although weaning typically occurs by 17·6 months, 23-month juveniles had only developed 68% of adult blood O 2 . Muscle O 2 was the slowest to develop and was 60% of adult values at 23 months. 5. We divided available O 2 stores (37·11 ± 1·49 mL O 2 kg − 1 ) by at-sea FMR (15·78 ± 1·29 mL O 2 min − 1 kg − 1 ) to determine a cADL of 2·33 ± 0·24 min for juvenile Australian sea lions. Like adults, young sea lions regularly exceeded cADL's with 67·8 ± 2·8% of dives over theoretical limits and a mean dive duration to cADL ratio of 1·23 ± 0·10. 6. Both dive depth and duration appear impacted by the slow development of oxygen stores. For species that operate close to, or indeed above their estimated physiological maximum, the capacity to increase dive depth, duration or foraging effort would be limited. Due to reduced access to benthic habitat and restricted behavioural options, young benthic foragers, such as Australian sea lions, would be particularly vulnerable to resource limitation.