The diving ability of juvenile animals is constrained by their physiology, morphology and lack of experience, compared to adults. We studied the influences of age and mass on the diving behaviour of juvenile (2–3-year-old females, n = 12; 3–5-year-old males, n = 7) New Zealand (NZ) sea lions (Phocarctos hookeri) using time–depth recorders (TDRs) from 2008 to 2010 in the NZ subantarctic Auckland Islands. Diving ability (e.g. dive depth, duration and bottom time per dive) improved with age and mass. However, the percentage of each dive spent at the bottom, along with percentage time at sea spent diving, was comparable between younger and lighter juveniles and older and heavier juveniles. These suggest that younger and older juveniles expend similar foraging effort in terms of the amount of time spent underwater. Only, 5-year-old male juveniles dove to adult female depths and durations and had the highest foraging efficiency at depths >250 m. It appears that juvenile NZ sea lions attain adult female diving ability at around 5 years of age (at least in males), but prior to this, their performance is limited. Overall, the restricted diving capabilities of juvenile NZ sea lions may limit their available foraging habitat and ability to acquire food at deeper depths. The lower diving ability of juvenile NZ sea lions compared to adults, along with juvenile-specific constraints, should be taken into consideration for the effective management of this declining, nationally critical species.