Over 3000 otoliths and 100 000 length and sex records were examined of southern blue whiting caught from commercial fishing grounds on the Campbell Island Rise, south of New Zealand, during August and September between 1981 and 1989. Ages of juveniles were validated by following modes in length‐frequency data over a 13‐month period between September 1981 and 1982. Ages of adults were validated up to at least 10 years by following strong year classes both from otolith‐based age frequency distributions and from length‐frequency data from 1981 through to 1989. Independent analysis of the length‐frequency data using MULTIFAN further supported the adult otolith ages. Above age 10 there was less confidence in otolith ages, less agreement between readers and a greater degree of bias between readers. When using the data in catch‐at‐age models it is recommended that ages greater than 10 be grouped as a single plus group.